I know I'm limited in my choice of chainrings because of the brazed on Fd mount. Can anyone tell me just how limited the range is? Will a 50-tooth ring work? I'm pushing 60 and don't think I can push a 53 for any length of time.

Here's the frame (nice, isn't it; 130-mm rear spacing):

https://page2.auctions.yahoo.co.jp/jp/auction/b148354689

Typically, a front derailleur braze-on mount is positioned so that the manufacturer's choice of front derailleur mounts in the middle on the adjustment range with a 52-53T chainring. Consequently, there is normally enough adjustment to easily accommodate a 50T. However, the dimensions of front derailleurs, specifically the position of the mounting bolt relative to the bottom of the cage, does not appear to be standardized. I have seen some difference between brands and years. It most likely will work, but there's always the remote possibility that you may select a front derailleur with dimensions that vary too much from the manufacturer's chosen standard. Selecting a late 1980s Japanese derailleur should maximize the probability of success.

These may help.

Here's the Shimano FD braze-on positioning guide for different size chainrings.



Here's a similar guide for Campagnolo.