I got a free 2005 Polaris fusion 900 with no motor can I swap a xc500 in it or even a triple

I highly doubt it, most likely you are stuck using one of the 700/900s from that Fusion setup. As those setups were completely different then the rest of the Polaris Lineup.

Only asking because I got a sledthst runs and drives I just figured if I ran everything from my other sled I could buy pass pretty well everything else

Like Coolhand said. The 05-06 900 (866 cc) and the 06 700 (755 cc) are the only engines that will bolt in to that chassis. If you have crazy mod skills, you may be able to fabricate a way to mount a different engine, but you will also need the electronics from the donor sled. The 900/700 is fuel injected and the electronics are all feeding the ECU for fuel/spark. So if you decide to tackle a transplant, make sure to get the wiring harness and electronics from the donor sled.

Yep, not only fab skills but a wiring genius needed. These new sleds just aren't great for transplant engines from other sleds.

Welcome
It may be easier to swap in something completely different
But as stated not as easy on the newer sleds!

Who would want an electronic engine? Just drop a Fuji 500 in it