Well you have yourself cornered there a bit then... OS or Evo and the OS's are finicky? (I have read that about the 91FX) Go Evo!
Any particular reason you do not like the G-90
I am leaning towards the Evo, but I am wodering if the OS still has problems such as air bubble development in fuel line from remote needle. There was a long thread started back in 2003 about the 91fx and its associated problems. Have most of these been remedied? I really would have sprung for the OS until I read a few threads and spoke with my friend. Although, there are good reports on the engine too. I guess I should mention that I may put a pitts muffler on the Toni to minimize the hack job to the cowl.
Regarding the ST90, again, I've haven't heard great things about the engines ever since the Italians stoppped making them. I have an older ST45 that runs fine, but I am skeptical about the newer ones. I know that's not a good reason to diss an engine, especially w/o personal experience, but I am reluctant to buy one to find out that it's no good.
I also noticed from the Toni manual that installation of the os91 may require some removal of the firewall to clear the muffler.
I should also mention that I have a Saito 91, currently in a Dago Red, but I wasn't impressed with the speed of the Dago and figure it wouldn't push the toni that much faster, therefore I'd rather go 2 stroke.