http://www.rcuniverse.com/forum/fb.asp?m=1171417
RE: Put OS50 to Rappy 30

Here is the link.

You probably do need to get a different mount, or if you have access to a machine shop you might be able to modify the one you have. (?) be sure that the distance from the top of the clutch to the mount stays the same between the two motors.

If you use the standard 30 gears ( 9.x:1) there will be little risk for overspeeding the head because the 50 is made for lower RPM thatn the 39. Most 50's run 8.5:1 gears.

The muffler from the 39 will not fit.

it is an expensive upgrade, but you will love the 50, i have 2, one from a 50 kit and one from an upgrade. both are a blast.