TOPIC: 1972 Starcraft Holiday 21 Deck Issues

I recently acquired what I think is a really cool boat, it's a 1972 Starcraft Holiday 21' with a 140 HP I/O. It's a beautiful boat but it's been sitting outside in the Oregon weather for years and there are soft spots in the deck. I have never worked on an aluminum boat of this size and am not familiar with what might be under what appears to be an aluminum skin for a deck or floor, perhaps some sort of foam? Does anyone have any experience replacing a deck on one of these boats that could offer an amateur some advice?

OK, Mercruiser 140. Parts are easy to find but no one will work on that old boat for you.

Here's my recommendation as a starting point:
1) Empty out & clean the boat. This probably includes removing the interior but not the floor at this point. Keep the interior for patterns.
2) Now you've removed the junk/rotten crap & gained access to the open boat. Now wash it all clean so you can evaluate it.
3) Those were good reliable & durable engines if taken care of. But if overheated, expect a blown head gasket, warped head & water in the oil.
4) If not properly winterized the block is probably cracked. Look above the alternator/under the manifold for a horizontal crack TYPICALLY ABOVE THE FREEZE PLUGS.
5) If not cracked but never properly fogged, expect stuck piston rings, low compression & lots of Blow-by.
6) Pull lower unit drain screw & check for oil, water, metal.
7) If all the above are OK you can think about trying to start it. Will need a new water pump impeller for sure. Most likely the fuel tank needs to be drained & cleaned, fuel fill, vent & feed hose (if not copper tubing as frequently used back then) all need to be replaced. Fuel pump is probably inop & carb all plugged up.
8) If it's a salt water boat, block & manifold may be rusted beyond saving.

I'm not trying to discourage you but don't start dumping money into hull restoration before finding out if engine appears to be good or at least runnable/rebuildable condition.

Now your question about the deck. You mention deck & floor in your post. I assume you are referring to the floor. I'd assume it's 1/2 plywood screwed to stringers. Rotten plywood covering saturated foam. If it's a salt water boat then heavy corrosion is possible too.

Did you find out why the boat was abandoned outside & uncovered for years?