I always thought Grunkle Stan would be right as Doc Brown, some people might say Ford would have been better but considering he actually came from 30 some-odd years in the future and the series already had a time-travelling episode "The Time-Traveler's Pig", it makes sense to put Stan in Doc's role. For Dipper as Marty is obvious so I won't explain because you should already know. Using the golf cart as a replacement for the DeLorean in this case is pure genius. Stan's car would've also worked. Seeing the flux capacitor in the back of the cart lets me know that Stan had to readjust the steering wheel in order to make room for the time machine settings.