I extended mine with used Yamaha parts
used MMax tunnel extension - stripped the paint to bare aluminum
used MMax hose covers
used MMax rear bumper
used Mtn Viper rear cooler
a couple of new Yamaha cooler hoses
and a new longer length blue Yamaha snow flap I got off Ebay for $17
I didn't need rail extensions as I'm running a 136 M10, but the TracksUSA ones are rated topnotch.
And I went with a 136" Camoplast Intense 1.5" - great track for on off trail. And I added a set of Simmons skis to balance the package out.
Tips: You can find these parts on Snowest or the Totallyamaha forum - I think I paid $200 or so for all the used stuff. ( excluding cooler )
The M Viper cooler, IMO, is enough cooler except for basically running ice, it has the right diameter hose ends to match up to the machine, and it easily fits in a stock MMax extension. And I'm running the 1.5" lug height. I sold my Viper ( SRX style rear cooler )and went with the Mtn Viper smaller one