kvfrider-Welcome to the forum!
Yes there was RER available in the late model 2001 MXZ's but if your sled did not have it than it is not as simple as changing out the MPEM module. I did it to my 01 Summit this summer and looking back, it was pretty straight forward. Here is what you will need:
1. First you need to pull off you starter assy and peek up into the upper left crankcase area were your pickup is located and see if the case was machined with the additional pocket for the second pickup. Most of them were but some were not. If not, you are dead in the water....if it is there, than you are in business.
2. You will need a complete wiring harness from a 2002 Summit or MXZ (ZX chassis) as they are all the same and wired for RER even if the sled didn't come with it. ***It has to be a 2002 model because in 2003 they changed from the 290W iginition system and then this mod would get more difficult***
3. You will need a magneto (flywheel) from an RER model (2002 only).
4. You will need a dual pickup coil from any 02 Summit or MXZ with RER.
5. You will need an MPEM from a Summit or MXZ that has RER and DPM and/or DESS (tether included) to match what your sled currently has. These items can not be added to MPEM so you need to ensure you find one that all ready had it. It can be off of a 600, 700 or 800 it doesn't matter. If you aren't lucky enough to find a used one from the same model sled as yours, you can send it to BRP race division and have it reprogrammed with the correct timing curve for your motor. Also, if the tether cord and cap were not included, you will need to have it reprogrammed for your tether as well.
6. You will need an RER secondary clutch with the correct helix and spring for your model.
7. You will need the RER switch/push button. Beeper is optional if you like to sound like a forklift when backing up. If you want the beeper than you will need a new DESS indicator light on your instrument panel or you will need to reverse the wires for it to beep. If you don't want the beeper, leave the wires alone and the DESS light will flash when RER is engaged. A DESS/RER indicator light will allow you to have both flashing light and beeper....your call.
8. Go on eBay and buy a service manual for 2001 and 2002 so you can compare the wiring diagrams when you do the wiring harness swap to ensure everything else is plug and play.
I know this sounds like a lot of work but once you have done it and look back it isn't all that bad. Since you are looking at doing this to an MXZ, you are way ahead of the game since 90% of all sleds with RER in 2002 were MXZ's and there are lots of people parting them out. If you are patient, you can find eveerything you need on eBay or on forums. I did mine for $225 in parts and $85 to have the MPEM reprogrammed.
If you decide to go this route, let me know and I can help you out with any questions you might have....