Other thing to consider is gearing related to tire size.
I was running a 50cc setup on a shorter tire and it ripped. Swapped the tire to a diff brand but same size with a higher rounded profile and I lost off the line gusto. 50cc is super sensitive to gearing as far as takeoff is concerned.
As well....,pulley size, and belt depth at zero variation is critical.
I swapped my stock moby pulley to an aluminum one that was slightly larger and the bike had much better launch