Any lawn tractor-type battery will work...smaller than an auto battery, but bigger than an ATV one.
Congrats on the find!! As long as it was stored correctly, you shouldn't have too much problem starting
it back up. 5-6 years is still pretty long for fuel stabilizer, so you may want to drain, and refill with fresh stuff. Look down a few posts for "parts suppliers" to look up oem oil filters. NAPA covers the 6800 engine
with NAPA Gold oil filter #1635, but I don't know if it's the same for the 3600.
Rattosh51
YT6800