On jets most any independent bike shop that works on dirt bikes, atv's, snowmobiles PWC, often have a selection of gently used jets from rejetting other machines. If you ask nice they might sell you a selection of jets to try and buy back the ones you don't use.
Most of these places often stock a wide range of new Mikuni jets for about the same price Mike's sells the Chinese stuff.
Compression, with the carb off holding them open makes no difference. 90 lbs. is low. A lot of auto parts stores have loaner tools, some rent cheap. Try another gauge. The book calls for on a fresh top end 145lbs. as minimum. It can be higher. On a good running engine with a few miles anything above 135 is ok.
Often times if the bike has set for a long time the compression will be low. A thin layer of rust can form on the rings, cylinder walls as well as valves and valve seats. This rust prevents things from sealing as well as they should. It wears off and lets the rings and valves seal better. After a few hundred miles the compression will improve.
I often loosen the valve adjustment a bit for compression tests. Say .010 intake and .012 exhaust, This ensures the valves are closing. Once testing is done reset to .003 intake and .006 exhaust.
You gauge may read low but are they even side to side. Even indicates your ok. Not great but ok. Testing with another gauge won't be a bad idea.
Exhaust, the stock headpipes are a dual pipe. The inner carries the exhaust, the outer stays a nice chrome color. When cut off as most do, the part that seals the inner to the outer pipe gets cut off. You can't just slap a muffler on the outer pipe. Exhaust will leak back out between the pipes and come out at the head. You need to hook the mufflers to the inner pipe. The adapter does this.
Condensers, yes the ones from a car will work just fine.
Leo