I recently purchased a 1990 Supra comp. it has a pcm ford motor but not sure if it’s a 305 or 351. Took it out and it ran fantastic except the temp gauge was bouncing all over the place and the alarm was going off from the moment I started it. I had a mechanic check it out (after waiting 6 weeks) and all he did was tighten the belts. Apparently they were ridiculously loose. I had been checking the exhaust when I took it out and it did get hot but never too hot to touch. The boat also didn’t miss a beat so I’m hoping I didn’t cause damage by running it as my mechanic said the belts being loose may have prevented my water pump from working properly.
Now I’d like to do some maintenance that my mechanic apparently didn’t think was necessary.

Any tips on changing out the impeller?
Would a loose belt cause the temp alarm malfunction? How would I check the temp gauge if the problem continues?
Is there a drain for the oil or do I suck it out the dipstick tube?
What kind of oil/filter should I buy and how much oil does it take?
Is there any other maintenance I should do?
What impeller kit should I pick and how do I figure out what motor I have?

All advice/criticism is greatly appreciated. Although I am somewhat mechanically inclined, I am a novice at best. I just can’t wait another 6 weeks for a mechanic who couldn’t figure out that yes I want the impeller and oil changed!