Disc brakes would require a new fork and to have a caliper mount brazed to the rear stays/dropouts. Also, your dropout spacing is probably 126 mm and all the disc hubs I know about are 130 mm (few) or 135mm (most). It's possible but not practical.

See if your fork and rear triangle/seat tube have clearance for larger tires. Racing frames, even 20 years ago, had pretty close clearances and may not accept tires larger than 25 or 28.

As to straight bars, a new stem, thumb shifters, and whatever else you want, they are all possible and maybe quite cheap if your parts boxes or the LBS's bottom drawers are suitably equipped.