I find that EBay is the best location for parts such as the flywheel and starter.
Last year I added electric start on my older Generac 6500w generator with Tecumseh HM-100 10 hp engine.
I did not add the charging circuit on it because it was very difficult to get the right flywheel and charging coil combination and since I want it charging while it sits I keep the 12V battery up by using a Battery Tender Jr.
One caution I will give is that you shouldn't buy a cheap starter. I bought a new Taiwan unit for around $35.00 and it puked twice in less than a year. The magnets would split along their lengths and then lock up the armature. With advice from a local small engine repair guy I found a NOS genuine Tecumseh starter USA made for around $50.00 and it works fine.
The flywheel you need is found on multiple engines 7~10hp and an old snowblower model would likely be the ticket.