You can get a 20t rear sprocket for some tomos wheels. You can also get larger sprockets for the front. Anything over 28t will require some grinding on the case to allow the chain to clear it. But it is the area next to the fly wheel so you can potentially cut into it without causing any issues.
The 70cc kit should get you up into the mid 40's pretty easy so there could be a number of things preventing that.
Transmission functioning properly? No drag in the gears?
Wheel bearings are good still? No drag on those?
Brakes are set properly and not grabbing the hub without the lever being pulled?
These are all easy things to check on which will definitely affect the speed of the bike.