1990 Supra comp

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!

On the transmission there should be a serial tag.
1st letter: P = pleasure craft marine engine
2nd letter: X = freshwater cooled
3rd letter: L/R = left or right rotation
4th letter: A = 5.7L 350 GM, D = 5.8L 351 Ford, K = 7.4L 454 GM, N = 7.4L 454 GM, P = 5.8L 351 Ford, Y = 8.2L 502 GM.
5th letter: transmission: P = pleasurecraft, W = Borg-Warner/Velvet Drive, H = Hurth.

I would think that if the belt is loose the water pump is not efficiently moving water and the engine would get hot.

The impeller is not hard to replace. The water pump is attached with two bolts. First, mark the orientation of the pump so you don't put it in upside down (this is easy to do). Undo those two bolts. Remove hoses. There are three screws holding the pump hosing together. Then the impeller comes out. Make sure all the vanes are there and not broken off. It usually slides out pretty easy with some pliers. Put some dish soap on the new impeller and it will slide in with a little rotation. You need to replace the O-ring at the same time. Skidim.com has impeller kits, oil filters, and fuel filters, and everything else you need.

There is usually a tube at the bottom of the oil pan that you can pull through the lower drain in the bilge and you can use that to change the oil or you can suck it out through the dipstick tube. The tube is usually clipped onto something by the fuel pump. Warm up engine before draining oil.

Lots of opinions on oil if you go searching this page. Seems like many like the 15w40 Rottella or Mobil1. I've been using VR1 racing oil SAE40, but it is harder to find. In needs about 5 quarts.

Thank you very much for your help! I’m really going to be bummed if I damaged the engine running it with loose belts, but I’m going to continue with maintenance and hope for the best. I didn’t notice any steam or funny smells and the water coming out was warm but not hot. Hopefully I get lucky there.