I have heard that non R/C engines such as the ones used in chainsaws ... for tree cutting and weed trimmers are useable in aircrafts with a slight modification.

I am being sold a Robin 50 CC engine ... it comes with the mechanical crop sprayer thing....will it work ?

What sort of eninge is really useful for such purpose ?
Do they really work ?
What size is good ? (the sizes available to me are 30 cc ----- 70 cc)

they are really really cheap here and i want to build a big sports plane for it.

Waiting for your generous help !
USMAN.