I would like to find a Switchback or RMK 144 800 VES. Are these motors reliable? I've heard that the exhaust valves need to be changed, is this true? Hows the fuel and oil usage? Any crank problems or bearing issues?

Any info on these motors would be greatly appreciated.

Yes, the valves should be changed to Stainless steel valves. They are available from Polaris for $90 per kit(you need two kits). The 800 has been known to have some crank problems.