The bypass should warm up a "little" but will never get "hot" because, well.....because it's bypassing. The only place it has a chance to pick up heat is at the "T" where it leaves the main flow and at the intake manifold that has cool air passing through it. The only time you will feel really hot water coming from the bypass would be if the outboard is overheating. I know on the later models (2000 up) if you hold your finger under the pee stream and then cut the engine you can feel scalding water come out just for a second as the engine comes to a stop. You COULD judge how fast the engine temp came up by using that trick. But, with the way the older motor bypass is routed I'm not sure that's even possible on those.
A good infrared heat gun is probably the best tool for checking engine temperatures. But even so, it's hard to get pinpoint results about the thermostat because of radiant and conducted heat in the metal of the block and head. The only true way to know would be a thermostat bench test....suspend the stat with thread in a pot of water with a thermometer in it and heat it up. I have used the trick of taking some of the thread and trapping it in the thermostat "seat". When you see the thread start to float away from the stat, that is the indication it has STARTED opening. After that there is a measurement of how far it opens called "lift" which is pretty hard to accurately measure since sticking your calipers into boiling hot water isn't fun and the thermostat starts closing immediately when exposed to air. So, it's an "eyeball" thing unless you can come up with a clever way to gauge it.
As far as "getting in there"...man! That's a crapshoot. I've found on the ones I've worked on, most come apart ok. But, I deal with mostly fresh water used engines. The one saltwater one I had was a corrosion nightmare and did have plenty of "missing metal". But I believe all the bolts came out no problem. 30 years is a long time so anything is possible.
If you can dose the bolts with "creeping" penetrant oils like PB Blaster or AERO KROIL (the best EVER) and wait for several days to a month, then your success rate getting it apart with no breakage will go up for sure. A running engine makes it a bit easier in that you can spray the bolts with the penetrant and then "thermocycle" the joints by starting it up and getting it warm. Do that several times IE: cold... spray... run until hot... shut off, wait until cold....repeat.... and your chances go up even more. A heat gun works ok here too. Just don't burn your barn down!
Good luck.
