I'll jump in. You might consider getting this bad boy....a lightweight 50 cc. This will haul the AgCat around with ease. I don't know of any 4 stroke that will haul a 25 lb. plane. On the other hand, if you try to build light and your plane ends up at about 20 lb. AUW you may be okay with that Saito.
BTW, check out the price of the XYZ 50, read A-F-F-O-R-D-A-B-L-E !
http://www.southeastengines.com/see_002.htm