FNG with a burned down 800 Mountain Cat

First of all, I apologize if this is too basic a question but searching never yielded quite what I was looking for.

I quit riding a few years ago and just got bribed back by a friend who found me way too good of a deal on a burned down 01 800 Mountain Cat w/ a 16" wide 159"

The motor has a stock pipe and can, 3 degree key, and head is shaved .030"

The sled supposedly burned down because one of the coolant hoses had a small hole and leaked out all the coolant, I suspect it may have something to do with the shaved head, timing key, and pump gas (91)

The sled has 2300 miles and I'm weighing my options for the rebuild, either way the cylinders need to be re nik'd and new pistons.

I am trying to build a reliable pump gas sled to get back into things but as we all know, stock isn't fun for long. Any build advice would be greatly appreciated (some harassment is deserved to for ever stopping riding )

Rumor has it 900 jugs and pistons are a bolt on affair, requiring only a set of 900 heads and pipe to complete it.

I've also heard rumors of boring it even further for m1000 pistons.

Should I just take out the timing key, put stock heads on it, and rebuild it to stock to start?

Could you guys shed a little light on my options for a reliable and affordable rebuild? The sled was $800 so I'm trying to keep this a fairly low buck affair but I also want to do it right and sometimes the upgrade is the same cost as a basic rebuild.

Thanks for any and all help :beer;

Idcatman1, thanks for the response. I would assume I have to go to 900 powervalves to match the 900 pistons? Also, what is pipe compatability? If I put twin pipes on the 800 will they also work on the 900 or 975?

Thanks again for the help!