An update. The YS 120 just isn't going to work. It would but only with some major modifications to the firewall and/or the cowl. It's well over an inch too long and it would look dumb with the crankshaft sticking that far out from the front of the cowl. I looked at several options like building a new firewall and mounting it behind the original then cutting the factory firewall out to clear the mount and carb. I also thought about adding some to the front of the cowl and blending it in but finally opted for a smaller engine. I ordered a Saito 100 today. It is about the same size as a 91 which I have and I could make it fit with far less work and changes to the original structure. It should be plenty of power. At least I hope it is. I'm not looking for 3-D performance but I don't want a dog in the air either.
Time will tell.
Tom