80cc MB motors aren't 80, they are 66cc, and have about all the power your typical bicycle can take.
The Grubee Skyhawk 66c is a good choice.
The 48cc has the same top end, the 66 just has much more torque.
Riding it on the roadway depends on your state laws.
For example in Arizona it has to be a 48cc or smaller motor to be classified as still just a bicycle. Anything larger makes it a motorized vehicle subject to having a drivers license, tags, emissions, all the safety equipment a motorcycle has, and insurance.
My next MB build about to start will be based on a MX bike and use a Jackshaft kit.
I won't go into how it works in this post, but yes, that is what you need.
Just do a forum search on Jackshaft or visit their web site, Sick Bike Parts, they have ad links right here on the site.
KC's Kruisers