>>>>> 1. No, there isn't a Stock Single or V Twin Engine I would just put on a Plane that makes 35-40hp. 2. Belt Reduction is Best. I haven't seen a Good Gear Drive Conversion yet, but may be possible. 3. Your going to have to figure that out, each Type of Engine will be different. 4. Yes, you want Electrics, with Electric Start, use a Lithium Battery. 5. Horizontal is easiest to convert. There is a 35-40hp@3600rpm EFI V Twin Engine's out there, but you still need to Modify them for Airplane use. READ POST #336 of this Thread. The Big V Blocks you need to lose as much Weight as you can. For Durability for these Singles & V Twins! 1. Valve Train, use Needle Bearing Roller Rockers, HD Valve Springs for Max rpm used, HD Valve Locks & Keepers, use HD Chrome Moly Push Rods, Billet Lifters, Stainless Valves. Even if using Stock 3600rpm, Upgrade the Parts. 2. Billet Rod. 3. Billet Cam 4. Billet Aluminium Flywheel. Stock Flywheel is Good for up to 5000rpm, but you Save a lot of Weight with the Billet Aluminum Flywheel. 5. Use a Good Full Synthetic Oil, like Mobil 1 ($7.88 qt) if Non-Filtered, means you have to change it more often. If Filtered I would use Mobil 1 Gold ($8.96 qt) 15,000 mile Oil, Walmart has it the cheapest in 5qt Containers. Some V Twins have Oil Coolers, I would use a Bigger one than Stock. Most Engines come with a break in Oil that needs to be changed shortly after Break-in Period they Specify. That would be a good time to Open the Engine and do the Mods needed for Airplane use, like Disable Governor, Disable Low Oil Sensor, Install Hi Rev kit, etc. Good Time to deburr and Polish all internal parts. 6. Use the different Engine Coatings to reduce Heat. 7. Take Out Heavy(4-5lbs) Balance Shaft & Balance the Engine Assembly. 8. Since most Airports only carry 100LL anyway, Design & Tune it to run that Fuel. Solves any problems from Detonation or Old Bad Fuel. A Vegas Carts 625 Single, $850 is 626.3cc and rated 23hp@3600rpm. It could make your 35-40hp easy with the mods I listed. The Small Block V Twins go up to 627cc 23hp@3600rpm! https://www.vanguardengines.com/na/...gines/small-block-vtwin-horizontal-shaft.html The Big Block V Twins go up to 993cc. https://www.vanguardengines.com/na/...engines/big-block-vtwin-horizontal-shaft.html Big Block Engines: Vanguard® 25.0 Gross HP* 896cc Vanguard® 27.0 Gross HP* 896cc Vanguard® 29.0 Gross HP* 896cc Vanguard® 31.0 Gross HP* 896cc Vanguard® 33.0 Gross HP* 896cc --------------------------------------- EFIVanguard® 35.0 Gross HP* 993cc Vanguard® 35.0 Gross HP* 993cc EFIVanguard® 37.0 Gross HP* 993cc EFIVanguard® 40.0 Gross HP* 993cc Vanguard® 40.0 Gross HP* EFI - Marine 993cc Compare these 993 Engines probably just different CAM used.