We did this to our 35 Ton Huskee a few years ago. It had a B&S 12.5 hp engine with recoil. The seals were going bad on the engine (several years old), and it needed an engine swap. For a few reasons, we decided to upgrade it to an electric start. The original didn't have any setup to retrofit a starter. There weren't any teeth on the flywheel, and no setup to mount a starter, etc. We ended up buying an electric start B&S 12.5 (essentially the same engine as the original with an electric start setup) and swapped it out. We mounted a battery box to the front of the engine mount on the splitter and bought a battery, a couple of cables and ended up putting in a key start so we could kid-proof it. The replacement engine didn't come with a gas tank (most don't) and some of the other essential bits were required off of the original engine to make it work properly, so take that into account when/if you buy a replacement engine as those parts may not be compatible or interchangeable across manufacturers. All in all, we dropped about $500 on the conversion, and have been very happy with the results - we're on year four with the new engine and run about 20 cord a year through it. HF may be your best bet for a new one. Otherwise, you could keep an eye out on CL for a used riding mower engine that you could use for the swap.