Brazing a new boss is a simple job for any framebuilder worth the name. It may also be worth getting them to make some forks worthy of the frame while they are at it.
Of course, the brake calliper is 'BB Sport' (and made of hexaminium or something).
Frame is about 24" seat tube (c-t) and 23" tt (c-c), with a 7&1/4" headtube. Doesn't feel too big for me though.
Strip down at the weekend.
Thanks
Mat
Looking at the DT brazings it looks as if the original owner was using a bar end lever for his front mech and a DT one for the rear, a quite common and fashionable option BITD. The DT boss was probably filed off to be able to fit a double clamp-on lever.