Crappy compression below 135 psi will give crappy performance. Compression should be at or above 145 psi. So you are going to have to do a ring job. It sounds like you are down around 120 psi.
If the crankshaft bearings are going out, you guessed it, more crappy performance. During the summer, if the sled had more than 3000 miles on it, you can take the engine apart and install a new PTO end bearing on a good crankshaft, piston rod bearings on the top end at the piston, and drill out the crankcase oil lubrication hole to the PTO bearing that the Polaris supplier decided to not install. And you are probably going to need a new front left engine mount because they go bad every 500 to 1000 miles.
If the bearings at the front and rear end of the track are shot, which they usually are at or above 2500 miles, yep, more crappy performance. The 3 rear idler bearings are easy to replace. But the front left driveshaft bearing replacement is a real bitch, especially since the drive shaft has extroverts. And don't forget to keep all the xerk fittings greased, especially that bad boy that is hidden at the front left driveshaft bearing. I just bought 2 sleds where the previous owners had not once greased any bearings or xerk fittings. Needless to say, the front left one disintegrated in flight on them, and they could not figure out that they had done anything wrong.
The spark plug leads get a little fried at the tip, but you can easily snip off 1/2" and screw the plug caps back onto the leads from the coil.
If it takes more than one pull to start it, then you get to rebuild the fuel pump by taking it apart and carefully flipping the round clear discs over and reinstalling them.
I concur on the single slp pipe for more HP, except a new one will cost you as much as the sled is worth at this point, since it needs rebuilding.
If you want track performance improvement, instead of power performance improvement, then ditch the Series IV track and get a Camplast Extreme with 3 lugs across that have stiffener ridges on them. Not the flat lug Extremes.