The rear derailleur hangar might be a simplex style which won't work with anything but a simplex derailleur. It's considered gauche by some to modify the frame. I wouldn't do it to a px-10 myself, but would try and use a claw-type adapter on top of the simplex one... but I've never wanted to switch my simplex RD out on my px-10 so I can't really vouch for the claw idea yet.

IRD makes great new freewheels so you can avoid cold-setting the frame if you haven't done it before.

Stronglight rings have a 122mm bolt-circle-diameter too, so if you aren't satisfied with the ring size you have right now you might want to change the cranks and I would recommend just using a VO sealed unit or if price is a concern, I think that Harris still sells a 20$ old-school french-threaded bottom bracket and then you could use any crank you wanted to.

I have a PX-10 that I kept in "as-found" condition and I'm quite happy with it. I have thought about seeking out some campy bar-end shifters but I doubt I'll go that route... I will eventually get an IRD freewheel so I can have a wider range of gears for my big self to get up hills. I'm also keeping an occasional eye out for smaller rings for the stronglight crank but am not holding my breath.

They are lovely bicycles and I think putting brifters and shimano on the frame might be more trouble than its worth, mind you my cleanup on this bike was a lot of effort too.