Having your seat retrimmed with modern foam will help with the sore butt, alternatively, I used to use a nice piece of sheepskin, Like those thick sheepskin seat covers you can buy for your car. Do you have those in the USA ? Stitch a couple of pieces of thick elastic to the sheep skin, cut to the size of the seat top, and just secure it with the elastic, works well, nice and comfortable , easy to put on and off....
We do have those in the US, so that would be a workable idea.
You can also get gel saddle cushions to go on top of the seat. They're available in black (and some other colors) and will take on less water if you get caught in the rain.
I'm working on a seat pad to go on an old (1970s?) fiberglass bum-stop seat pan. The plan is to use a chunk of ~2" thick neoprene cut from one of those garden-kneeler things that older folks use. Once the piece is cut, I'll cover it in marine-grade vinyl seat material. Not sure how comfortable it will be but it's not going to be on a touring bike.