shorten the throttle and clutch cables to fit so there's not excess cable hanging all over the place. (there will be if you don't shorten them).
you will have to noth the rear fender for chain clearance, and also cut about 1/3 of the left side brake pad off for chain clearance. you'll have to modify the rear brake caliper for chain clearance also. I raised my caliper with spacers & i actually bent the caliper as well.
you'll have to make a custom pipe for the best fit.
you should add chain adjusters to the rear drop outs because the way the drop outs are, the torque from the engine chain will want to pull the rear wheel forward,
the chain adjusters will help hold the rear wheel from being pulled forward.
for the best look, make a custom tank.
the stock tank can be used, but they look hideous on the occ in my opinion.
the occ seats are terrible too, so for the best result, make a new seat post and put a more comfortable seat on it. the stock occ seat is VERY uncomfortable and makes the bike almost impossible to ride for more than 5 minutes.
chain alignment is an absolute must, especially with the aftermarket engine mount. once you drill those holes in the frame, you're committed, and if your measurements are off, the chain will never be aligned right.
the mount i used is slightly adjustable side to side, not sure if the one you got is or not, but the side to side adjustment is very minor.
you can buy chain links on e-bay cheap...like $5.00 for 15 links, you just have to search for them.
the stock rear caliper brake SUCKS, and will make it difficult to stop the bike.
I mean, it will work, but the stopping distance is terrible.
building an occ take a lot of planning and modifying.
i hope you didn't expect to just bolt the engine on and go.
the best advise i can give is to be patient, take the time to build it clean (hidden wiring, shortened cables, custom chain tensioner (mine is hidden), relocate the cdi to hide it, add an automotive spark plug wire and an ngk spark plug, high flow air filter, custom gas tank, better seat and add a front brake (if possible). my occ doesn't have a front brake and the forks don't have the bungs to mount a caliper brake to it (some did come with front brakes).
a 44 tooth sprocket on that 20" wheel (don't forget, the rear tire is 24 inches tall which will give you a little more speed) but you'll only get about 25 mph out of it. (depending on rider weight)
i have a 41 tooth on mine and i'm getting right at 30 mph (but i also built the custom exhaust, my carb has been re-jetted and i weigh 150 pounds).
i think the best sprocket for one of these is a 36 tooth.
here's what mine looks like.
currently i have the gas tank off because i decided to re-paint it AGAIN.
here's what it looked like with the tank painted black (the first paint job)