Planning to install an 800 in my 03 Rev 440 chassis. This is just the start of the project, i have not even picked up the engine yet. The crank and case are a 02 std 800. The engine now has a new top end from a 05 800HO. The sled will be used for mainly trail and ditch riding, around sea level in central to Northern Minnesota and the western U.P. I plan to use 91 min. octane which is readily available. The mill will be set up with stock Ski-Doo flat 40mm heated carbs and DPM. Exhaust will be the stock 800 or 800 Ho pipe which ever is needed. 1). I know the junk Doo pistons will go but should i use the 800 HO cylinders or the std 800 cylinders? I do have a MPEM from a 02 800X. 2). If the cases will not allow me to add RER i may use this MPEM, can i use this with the ho cylinders?

yes you can use the h.o.cylinders on the 02 base with any 800 rev pipe.
also if the oem pistons don't have many miles on them , just install new rings if needed.... oem pistons are not as bad as the web makes them out to be.......I have seen many high mileage 01 and 02 800's with no piston failures.......