There is not a MAJOR difference between 2 and 4 strokes. Read the manual that comes with any engine you buy and you can't go wrong. Yes you can get parts for a .91fs and no 150 is not a good price if you compare it to these auction's.
http://shop.ebay.com/i.html?_kw=os&_...ts_Accessories
I quit dealing with tower a long time ago.