Sounds more like a Ryobi 31cc engine. I have one and that is just what it looks like. If the Mac 31cc weedeater is a Ryobi look alike you can get parts from Peter Faith at ww.wackerengines.com. Peter is a great guy to work with he treats all of his customers riight. The Ryobi weighs in at 4lbs and is a long narrow engine. It will turn a 18x8 prop at 7200 RPM and the is enough to do some nice comfortable sport flying on a 14 to 16 lbs bird. The conversion is very easy and almost anyone can convert one of these engines with Peters parts. I will cost about $60 convert this engine. That is very cheap and it will be a very reliable engine.
Thanks