Josh Burrup/Sinister Sand Sports

Americans tend to think of the Smart Fortwo as a fuel-frugal, docile ride, but a few mechanically inclined enthusiasts are tinkering with, if not wholly upending that image.

Fabricators, primarily outside the United States, have made a pastime of mating the body of the tame eco-cart with the engine of a Suzuki Hayabusa sportbike. The results can be seen on YouTube smoking tires and humiliating Ferraris in drag races.

Though these crossbreeds had appeared primarily overseas, at least two American firms have built prototype ‘Busa Smarts and extended their expertise to like-minded motorists, offering conversion kits and complete build-ups.

High-octane Smart cars are not so new. Daimler, the parent company of Smart, has marketed a skunk-works version, the Brabus Smart, with a turbocharged 1-liter in-line 3-cylinder engine tuned to about 100 horsepower, about 30 horsepower more than the basic Fortwo. Hayabusa-powered Smarts, in contrast, generate 170 horsepower, or 10 more than the Fiat 500 Abarth, which weighs about 1,000 pounds more than a stock Smart.

“I’ve driven plenty of Brabus cars and turbocharged Smart cars, and nothing is even close to the Hayabusa Smart,” Matt Boesch, a Michigan native who named his Smart-Hayabusa amalgam the SmartBusa, said in a telephone interview.

But that singularity makes for a self-selecting niche product. “It’s fairly temperamental,” Mr. Boesch said of the SmartBusa. “It’s pretty hard to drive. If you put the pedal all the way down, it would kind of wheelie. It will sort of pull the wheel from your hands.”

Mr. Boesch would gladly accept a partial loss of steering, however, for the respect earned when he takes the SmartBusa to events like the Woodward Dream Cruise, the annual cruising-season event in Detroit. “When you step on it, it would shoot flames up the tailpipe and take off screaming like, well, a Hayabusa,” Mr. Boesch said. “It was instant credibility.”

The conversions are more common, though hardly commonplace, in markets where Smarts have been sold for over a decade. Mr. Boesch noted there was, tragically, an ample supply of salvageable motors from crashed Hayabusas.

Mr. Boesch carries on a tradition of fringe fabricators in Michigan. Just last week, the state’s motorsports community mourned the loss of E.J. Potter, otherwise known as the Michigan Madman, who wedged a small-block Chevy V-8 into a motorcycle frame to create his signature bike, the Widowmaker.

Mr. Boesch described his first SmartBusa build as part experiment, part dare. “Somebody had done something similar in Europe, and everyone on the forums was talking about who would be the first to do it here,” he said. A former Nascar fabricator, he currently manufacturers aftermarket parts, including an exhaust for Smart cars, through his company Boesch Built Automotive Fabrication. He created the one-of-a-kind SmartBusa for a friend, but subsequently had to talk potential buyers out of hiring him to perform similar conversions. “I try to discourage people if this is going to be their only car,” he said. “It’s kind of a toy when it’s done.”

He briefly considered selling the assemblies and instructions as a kit, but hesitated. “I think the customer service phone calls would be way too much,” he said.

Josh Burrup, of Oklahoma-based Sinister Sand Sports, has no such qualms. Mr. Burrup, who goes by the name Yoshi, sells a full Hayabusa frame and suspension conversion kit for roughly $16,000, though the price varies depending on modifications. For another $4,500, he will add a Hayabusa engine modified with a turbocharger to generate more than 300 horsepower. “I have shipped two kits and have four more being built,” he said.

Mr. Burrup’s full-time trade is in fabricating custom sand rails, which are high-powered, tube-chassis dune buggies. He sells his Smart kit as a full subframe that widens the Fortwo’s stance about six inches and, he claims, improves handling with a custom suspension and 17-inch wheels, all of which bolts to the Smart’s unibody. The custom rig trims 200 pounds from the stock car’s curb weight, he said.

His first prototype was built for a buyer in Canada. “I had a guy call me and wanted to know if I could do a Hayabusa in a Smart car. I said probably,” he said. Mr. Burrup did not have it in his shop long enough to burn through a tank of gas. “I drove it around the block,” he said. “Yeah it sounds awesome, like a street bike. It’s a fast little car.”

Since that commission, he has fielded a steady stream of calls, mostly from Canada, he said. “The most interesting query I got, they were shooting a movie overseas and they wanted six car kits,” Mr. Burrup said. “The big thing is they wanted it to look stock, but I don’t think they are safe at all stock.” Though he did not get a commission, Mr. Burrup noted that the British firm Smartuki Motorsport later built kits for Smart Roadsters in the 2010 film “Knight and Day,” starring Tom Cruise and Cameron Diaz.

Mr. Burrup said his conversion business was growing, to the point where building sand rails might have to take a back seat. “I think it is going to take over my other business,” he said of the Smart builds.

Mr. Boesch, meanwhile, plans to perform conversions in the down time from his primary business. He said he had doubts about the long-term sustainability of the conversion business.

“To get someone to buy a $20,000 car and spend another $20,000 to basically ruin the car is a little insane, really,” he said. “It is very hard to find people who want to do that.”