ok i just recently bought a plane from a guy and he had a os 91 four stroke on it I was wondering what 2 stroke motor would be equal to it

A .61 two-stroke would deliver similar power.

Something close in displacement. So a two-stroke between .75 and .90.

OS 61 is going to be the closest equivalent.

Ken

I just replaced an os 91 surpass 4 stroke with a 65 2 stroke. Flies the same.

On weight differentials - the OS 91 FS weighs in at 24.2 oz and the OS 61 FX is 23.6. Also, both of these OS engines share the same mounting foot print - meaning that the OS 61 can use the existing OS 91 FS mounting holes. The OS 61 FX is a bit longer (4 mm)  than the FS - no biggie. Throttle control rod location may well need to be relocated.

Have fun with the conversion.

I'll throw in a vote for the Tower 75. Same weight as the OS 61, but with more power and about half the price.