In order for your engine to output enough torque to power your bicycle, we will need significant gear reduction. We will be using a jackshaft for 2-stage gear reduction in order to avoid having to use gigantic sprockets. For a bicycle with 26" tires, I suggest gear ratios anywhere from 1:11.5 (speed) to 1:15 (power). To give you a baseline, my current favorite bike is geared at 1:13 and has enough torque to go up pretty much any hill without any pedal assist, and can take off from a standing start, but can still reach speeds of 36 mph on level ground. If you gear for speed, you can hit over 40, although with an 1:11.5 ratio, you will need quite a bit of pedal assist to make it up the hills. In my opinion, 1:13 is a really good balance between speed and acceleration.

Now, on to how to calculate your gear ratios:

For a single stage ratio, simply count the number of teeth on your drive gear, and the number of teeth on your driven gear, and write them as a ratio (x:y). For example, if we have an 11t sprocket driving a 36t sprocket, our ratio is 11:36, or about 1:3.27.

Now, we are using a 2-stage gear reduction, so things get a bit more complicated. For a 2-stage reduction, the overall gear ratio is the product of the individual stages. So if you have a gear ratio of (a:b) for the first stage, and (x:y) for the second stage, your overall ratio would be (ax:by). For example, let's say we have the 11t driving the 36t from before, but then the second stage is a 9t driving a 36t. First we multiply 11 x 9 = 99 and 36 x 36 = 1,296. Then you write them as a ratio 99:1,296. Then, we simplify that ratio by dividing bot sides by 99 (divide the left side by itself, and the right side by the left side). So 99 / 99 = 1, and 1296 / 99 = 13.09. So the final ratio is 1:13.09. In case that's too complicated for you, here's a link to a Gear Ratio Calculator.

Keep in mind when ordering sprockets, the first stage uses #35 chain, and the second stage uses #41 chain.

The first sprocket will probably be an 11t, because as far as I can tell, that's the only tooth count that you can get for a max torque clutch with a 5/8" bore for a #35 chain.

If you want a 1:13 ratio, use 11t (max torque clutch, 5/8" bore, #35 chain) - 36t (first stage driven sprocket, #35 chain) - 9t (second stage drive sprocket, 5/8" bore, #41 chain) - 36t (second stage driven sprocket, attaches to rear wheel, #41 chain).