The 725R is the long shaft.

The 700 is the short shaft.

The ones from Harbor Freight can be had for around $50. They state "factory reconditioned", but my engine looked perfectly new to me. Cost me all of $58 shipped to my front door. I bought the 725R engine in an MTD/Yard Machines weedie. It is a twin ring with NO compression relief slots. Carb is a little small, but anyone who tinkers with conversions is gonna have 5 or 200 carbs lying around.

You can get parts from Peter Faith at www.wackerengines.com I heard he got hit by a hurricane, so his website might be down. Don't have any idea what type of business he's doing after the storm. He's a good guy and will talk your ear off. Full of knowledge.

www.carrprecision.com is another place to get conversion info.

Go to ebay and buy "parts" chainsaws. As long as the cylinder and piston ain't scored--you can pretty much make an airplane engine out of any of them. Try and stay away from the ones with needles on the crank bearings--McCulloch in the smaller sizes. They can be made to tturn a prop--but it's a bit more work. Easy if you have a milling machine and a lathe--not so easy if you ain't got those tools.

Chainsaws are kinda doggy on power untill you get up over 35cc. Get yourself a 40cc or 60cc chainsaw engine and you'll be pooping your shorts when you start it for the first time with a big hunk of lumber bolted to the business end. It's a kick in the pants!!