I've painted my motor 2 times in the last 10 years, just using rattle cans. Silver both times with a clear coat. I used Dupli-Color Engine Enamle 500 Degree Silver and Clear the first time . The second time checkers was out of the clear so, I used Rust-Oleum Painters Touch Ultra Cover 2X Gloss Clear. Seems to be holding up fine.
The silver needs to dry overnight so, when you apply the clear it doesn't mottel (that's when the clear re-wets the silver and makes shadows).
At any rate, after a couple of years it starts bubbling from oxidation even after sanding and dremeling the nooks and crannies with a wire wheel.(Brass)
I like so, many to get back on the road have done rush jobs, even knowing how to do it properly.
In the 80's I worked stripping and painting small aircraft. Mostly Piper's and Cessna's.
So, if you have the time, patience and want a paint job that's going to last 10 to 20 years (maybe longer), here's how to do it.
Strip the engine with paint stripper. Seal the engine by plugging the intake and exhuast with rags and then silver duct tape them because stripper won't effect this tape but, it will loosen the glue and get under it unless you use a bondo squeegie or something similar to press it down tightly. Tape off the stator wire harness, shift shaft seal (I would replace the seals with new ones after it's painted just to be safe), clutch seal, cam shaft seal, etc. Overlap the silver tape about 1/4" onto the painted suface. Use a water souluble stripper so, you can wash it off with a water hose. I would not use a pressure washer. That could blow stripper into the engine. Apply the stripper to the engine and let it stand for 10 minutes or so. Then lightly scrub the engine preferably with a wood bristle brush, wire brush or something the stripper won't melt. Apply a second coat of stripper, let stand for another 5 to 10 minutes, scrub again. If you have any dry areas apply a third coat and let sit for 1 minute or 2, (if the stripper dries a new coat will reactivate it). Then spray it off with a water hose. Use a brush or scotch brite when rinsing to remove any residue. Don't hit it with water between coats or you'll neutralize the stripper and have to wait until the engine is completely dry to apply additional coats.
If you have or have access to a sand blaster it's the most efficient and thorough way to remove any corosion but, it's risky. You have to be careful and stay away from the seals and any other areas such as intake, exhuast etc. Unless of course you disassemble the entire motor.
The other way is after it's stripped is to use sand paper and a dremel to remove the corrosion.
After stripping, take a single edge or utility knife blade and cut the tape back to the seals or onto the seals so, you can take 220 or 320 grit wet and dry sand paper to remove the paint that wasn't stripped. Now it's ready for the aluminum prep. DuPont 225s and 226s. Read the aluminum prep instructions thoroughly as I think the window is 24 hours to get primer and or paint on the aluminum before it starts corroding all over again. It's a 2 part system which is an acid etch wash which you apply with a scotch brite, scrubbing it into the aluminum to clean it properly. Apply a second coat (with a spray bottle or wooden throw away brush). Then rinse well with water. Make sure it's thoroughly rinsed. Now apply the second stage which is an alodine solution. This seals the aluminum. Do this immediatly after rinsing off the acid etch sollution. Again, make sure all the acid is washed away. It's like washing dishes. There will be bubbles until it's thoroughly rinsed.
If at this point you discover you have missed some corrosion. Allow the engine to dry or blow it off with compressed air and then use a brass brush or wheel on your dremel and remove it. USE ONLY BRASS, DO NOT USE STEEL. Steel will create electrolosys and start it corroding immediatly. Blow off the motor and or hose it down agian.
Blow it off or let it dry and paint away.
If you decide to go all out and tear down the, motor you can always powder coat it.