Sheldon,
I read somewhere that the conversion is .6 to 1. (2vs4). i.e. a .61 2 stroke would need a 1.00 4 stroke to be equal as a rule of thumb.
My personal experience is I'd take a YS 91 over a .61 2 stroke any day.
(Keep in mind we own 6 YS-120's and 2 YS-91FZ's,,,in addition to the big gas engines)
FWIW,
Steve

Puckett Model Aviation