I have seen pictures of sleds that caught on fire. Everything burns down to a little pile of ashes in the snow. About all that is left is the crankshaft, shock springs and small steel parts.The track and seat burn so hot it turns the tunnel into ashes, and all the plastics and gas burn so hot it melts the engine and bulkhead. You could put all that is left in a couple of 5 gallon paint buckets.
Nothing even left to salvage from your ATV. Hope you figure out how it happened and let us know so we may be able to prevent it from happening to others.
Hope you have good luck with your insurance and get a new ATV without to much trouble.