I am late to post to this party... Ha... I have not really considered a 1992 bicycle an old bike but looks like the time has come. So now it can be hard to find the proper seat post size on a 92 era bike. How time fly's. Just think of trying to find the seat post size of a 70s era bike after you have salvaged the frame from a junk pile. Yep, that's a challenge. Even if ya know what seat post is supposed to go into it. Here's the way I do it and remember its not "The Way but just A Way..."
Clean it up and sand it down inside and out
Give it an eye ball top to bottom
Is it round, inside diameter lower, inside diameter upper, fix what you can
Get a pipe and wrap tape around it to get the diameter that fits comfortably upper and lower
Use that diameter to get or make the seat post you need
Using sand paper to adjust the diameter of your seat post is no big deal if the seat tube has been properly cleaned and prepped
Don't forget to use a liberal amount of quality anti-seize compound from top to bottom on that seat tube before your final assembly
Hope to see you on the forum as ya go along and thanks for educating me that bikes made in the 90s are now having the same problems as those made in the 70s...