its the OSMG2104 - www.osengines.com/engines/osmg2104.html altho i am using the slide carb version, doesnt make too much difference i guess, but the rotary is supposed to be more suited to offroad, and uses a slightly more straight forward linkage.

it drops straight in mate, u may need to shim ur pinions a bit tho.

seems odd that the cen 18 doesnt work right... i dont have any experience of the 18 conversion, maybe someone out there can shed a bit more light on it before u pay for the os...?