I'm impressed that you scrounged and installed an engine in such short order. Great job.
Ensure that the shaft is properly aligned at the transmission or you'll wear out the shaft log, stuffing box and cutlass bearing in a short time.
Regarding through-hulls- Pearson did not use gate valves, at least not in the P-30's. I was hull #255 and I visited one of the #100's with original gear and they were ball valves or Groco rubber cone seacocks (for the toilet flushing water and discharge).
The galley and head sinks are metal "standpipes" that are heavily glassed into the hull. While this is not ABYC-compliant these days, I found mine to be very robust and eventually I quit worrying about them. On a port tack in heavy weather, sometimes a little seawater will back up into the head sink but it doesn't overflow onto the floor or anything. Just don't store anything large and heavy near the standpipes that might impact them and you'll be fine.
I see that the compression post issue has already been noticed and addressed. Eventually, you need to get something under that post of the cabin top will "dish" inward, around the mast base. G10 (Garolite) from McMaster-Carr is a great product to create the shim from. It has incredible compressive resistance and is rot-free. Make a shim, whack it in place, glass a tab around it, and get on with life.
You seem pretty handy. If possibly, get a hydraulic automobile frame-straightener or a bottle jack with a long, VERY strong rod and jack up the cabin top near the top of the compression post (use a large, wood block to protect the cabin top liner) to make it easier to get the shim in under the post. You'll need to slack the standing rigging before you start, or just remove the mast. You can tell you're getting things lined up with the door striker gets close to lining up with the hole.
Check this out for ideas: https://pearson30olvido.wordpress.co...t-replacement/