My first Fixed/SS mini bike project (and other infmz projects)

I can see that there's not been any extra input from forumers re possible solutions to your fork problem, unfortunately. My idea was that possibly the TT calipers could be designed to get the calipers closer to the tyres fitted to wheels, and so would be curved on their undersides give extra room for tyres to pass without catching on them. But I've had a look at one of the TT brakes I own and the underside isn't designed any differently from any other caliper. The main advantage of the design is that they can be compact little items and so more aerodynamic and powerful, but only when used with tyres having a tiny width. For bigger tyres (like you'd like to use- 25c plus), and forks positioning the wheel/tyre more than, say, 1-2cm below the underside of the fork crown the calipers just won't work. It's unfortunate for you though, as this idea could have been the last possible 'save' for your problem. The calipers seem to make it clear that it's not just these calipers that wouldn't help with the issue of using wider tyres, but the basic design of all calipers must be one where the designers don't think about the issue some riders have of using a fork that only allows minimal tyre clearance. So I don't think any caliper invented will help you out. Basically, what it boils down to is getting a new fork (or the brake adapter mentioned previously), and even with this option making sure the new one is correct in it's measurements.