I would say go with an e50 for the time being.
Fast za50's are not any harder to make compared to e50's but solid ones are another story. Expect to pay a big premium if you want a two speed that will last.
My last ZA80 metra lasted around 1k miles (including 200 miles of blasting at the latebird rally). Every last thing on that motor was build to as tight of tolerances as I could get, in the end the brass bushing on the crank was the weak link. It's failure cost me a piston (thankfully the cylinder was not shot). Roller bearing con-rod+having a machinist press in + a new piston was about $175. Next thing that will probably happen is the magneto side crank shear but hopefully I will get another 1-2k out of it before that happens.
E50's are stupid easy to build right, with experience you can put one together in a few hours and have it running on the bike.
With a properly tuned clutch the correct gearing and pipe your e50 will accelerate just fine and hit very high top speeds.