Its a tough one! Engines aren't the easiest of things to scratchbuild, i've not attempted it yet. Having said that, i guess they would be pretty straight forward if you have a shed load of reference pics of the engine broken down into its main components. The hard part is finding them!
Anyway, i suggest contacting the guy who runs the Yahoo Rally Car Model Group - Nenad is his name. He was building one some time ago and had a few pics on the site, which have now gone.
http://groups.yahoo.com/group/RallyCarModelGroup/