Re: Side pull calliper brakes?
As long as the mechanical parts line up, there is no reason you can't use the hole in the bottom of the fork to mount the brakes.
I realize that mounting disc brakes can involve some custom work/welding but if you have the tools and skill, disc brakes are far superior to the rim type friction brakes. Whichever you decide on, the addition of a brake on the front wheel is highly suggested.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"