Rear spacing will be 126mm

Oddities to look out for . . . . are 21.1mm inside diameter steerer tube. Also on the canti studs, some early MTBs put the anchor point for the end of the spring on the outboard side of the stud, which limits your brake choices (though there are still good ones).

If it's a Japanese early MTB then the BB will guaranteed be 1.37 x 68mm, English. World of choices.

Seatpost size worst case scenario could be 25.8mm, which is somewhat scarce in the vintage market, but easy enough to buy a modern Kalloy 25.8mm.