Azusa will make a custom aluminum sprocket to your specifications. Get your specs right because I doubt you'd be able to return a custom made part.
http://azusaeng.com/product/custom-aluminum-sprockets-blanks/
"FINISHED" sprockets come complete with
bore and bolt circle pattern precision
machined pursuant to the pattern
you specify, ready to mount.
FYI: An odd number sprocket tooth count will provide more even sprocket and chain wear and thus longer life than an even number, probably not too important if the bike won't be ridden much.
A sprocket hub/adapter that accepts split sprockets would be nice, if you can find one to work with your wheel. There are so called "universal" ones but they aren't going to fit everything.
Engine plate: Replace the plate, use plate thick enough that it won't flex much, flexing will cause the chain to go tight/slack as the engine moves/shakes in addition to other potential problems. Another option would be to weld the holes in the existing plate then drill new holes. A no-weld option would be to drill new holes in the existing plate and use thick fender washers on the bottom of the plate. Fender washers have a large OD relative to the ID so they cover more surface area than regular flat washers and will reinforce the plate around the bolts. Or, instead of washers, you could put a plate underneath the existing plate; the Azusa heavy engine mounting plate is nice and pre-slotted, might need to trim it to clear the frame tubes. As it is, the engine sitting on 4 nuts rather than a flat surface will put a lot of stress on the mounting ears of the engine block, they could break. It would be much better to have the engine on a flat surface, as it was designed to be.
After all of that, I had a better thought. If you were to add a jackshaft, you could mount the engine using the original slots. After replacing the rear sprocket with a smaller one, you probably would need a jackshaft to achieve a decent ratio anyway.
