Howdy,
I have an opportunity to get a cheap 2006 RMK 900 with a blown rod (or something, haven't looked at it myself yet).  I know the 900s were discontinued because there was to much vibration and that's why Polaris was always updating the engine mounts, but is it possible to rebuild a 900 and get the right mounts and have a bomber engine?  
If not, is it possible to make that engine a 700 or an 800 and be good to go from there?
Thanks a lot,
Matt

Possible, yes.... but really, unless someone's giving it to you for free, you're probably better off looking at an '07-09 dragon.  

The 900s are great machines, I've got one in my garage right now... but they need a lot of work to get them to the way they should have left the factory (Motor mounts, sway bar removal, new skis, replace the front bumper, front radiator delete, new throttle/oiler cables...).  

Colin@CompFusion (email here) makes a great motor mount kit... replaces the stock ones, drill two more holes through the engine brace and bolt his mounts up.

However.... doing this, the vibration/torque's still goign to go somewhere... like into the engine brace.  I have two broken ones in my garage right now, designed a new one that's solid aluminum and so far it's holding up well (the urethane pucks, however, are not).  I'm pretty much at the point now where I'll just remove the motor and replace the urethane pucks every year.

Even if you get it running great, it's still a 6 year old sled that weighs a lot more than the dragons do.

Thanks for the info.  I think I'll stay away from those.