Hi,
I have a couple XT-90 trimmer motors. One is still a trimmer and the other is just the unconverted head. They are about 30cc. I beleive that the XT50 and XT20 are much smaller versions at about the same weight. The design is very much like a Ryobi 31cc with reed valve and exhust in the rear. The exhust is round exit and the muffler is held on with springs. The muffler can be modified for R/C. It may be a little heavy for its power. The crankshaft stud should be 5/16x24 thread which is easy to get. Same as Homelite 25/30cc. Here is the website for parts.

Dave

http://www.odref.com/poulan/pdf/WEED...MERS/xt-50.pdf