The size of the vm20 on your 50cc setup. You already have to tune them way down for a 70cc bike.
Get a bigger Sha tho and have some fun. To me, a Sha is so much easier to tune since there are fewer "stages" to it so less tuning options in a sense which makes it a little less daunting to get tuned up.
Grab a 15sha and accompanying intake. Get some jets in the high 50's thru mid 60's range.
Upgrading the pipe will help a lot as well. Ask questions as you have them.