yuck.. that's the biggest job there is to do to one of these sleds, I don't ever replace the front heat exchanger unless its got a gaping big hole in it that cant be patched, only have ever had to change them 2 times,( I learned real fast not to change them out...lol) I weld them. As to replace you need to remove engine, remove steering stem, remove rear suspension, track, etc. and then you got about 50 rivets to drill out and more then likely need a small porta power to get it out of the already flexed aluminum chassis....it stinks!!!
I remove rear suspension and track, then wash it out well with water to get rid of antifreeze, take off the two front hoses from heat exchangers to flush well and leave off to get excess water back out, you can tilt from side to side to drain it good.
I drain the gas from tank and run engine till it dies from no fuel. Then hang the sled up by back bumper as high as you can possibly get it!!! Preheat your area of the hole well with map gas torch, I'm talking smoking hot to get the water around the area your gonna weld, its drawn to heat. tig weld the hole closed and then reassemble. Still is no fun but about only 30-40% of the work and labor cost to change it out.