That Tecumseh on the freebie, if you can get a cowling and pulley and starter off one of the others with the top starter it will go on.
Then you dont have to mess with the side starter.
I had one here with the rear mount tank (with the reversed engine on the rover rapid it was on it was actually forward mount tank) and i used one of those tanks so it had a side mount tank on it.
As long as its the same shape cowl.
That Ultima. They are a bit of a pain in the butt thing. I had one here and it was a bit of a pain. Turns out the bottom seal was leaking, i new that, but the blade plate was loose. I never checked that as i wanted it running before i spent a cent. Thats a good theory with them.
When they are working right they are a good thing.
I put my ultima engine on a late alloy base. Got good coin for it seeing as its winter. Then put a PT on the silver base and called it a silver streak. Got fair money for it too.