[sho305]

The Vanguard has the most parts available, so anything you want to do in the future is easy to get. If you don't have some other reason, I'd say go that way. The 18hp is cheaper and it will get great rpm for high speeds with just a rev kit, that is why they are popular. You can put a 23hp cam in cheap maybe larger carb I don't know what they go for, without even going aftermarket. The 23 is going to have more torque and if you build it up later more power overall and can be made to run more rpm. So stock the 23 will have the edge pulling big loads up hills, on the flats the stock 18 and rev kit will run fast. If you want more later the 23 is just plain bigger engine to mod unless you jump to the 35hp model. I would put a rev kit in anything, the vanguard one is not that much and you don't have to worry about damaging something when run without a governor. I just bought better valve springs for my cheap clone engine with stock cam in it.

If you have stock power, any of these twins will have a LOT more power than you have now. Even a 15hp clone really feels/runs like twice the power.

If you end up later putting a cam in your engine, you will want an aftermarket clutch. Unless you can deal with your clutch most Yamaha you can't do much with them. A cam will need more rpm to get full power up a hill or takeoff power.