The 924 CGT (937) used the same brakes as the 924 Turbo ( + 944 / 924S).
Id look at Boxster monoblock calipers, whichll fit with just a machined adapter. The only other 'simple' choice, although actually far harder and more involved, would be to source 1986MY 944 Turbo brakes / hubs / etc and fit these. Theyre inferior, hard to find, will need much restoration and will involve a lot of work to fit; so Id go for the Boxster option myself.
There isnt a plain bolt-on upgrade, in answer to your question:although the Boxster caliper option is pretty close. As others have said - the original brakes arent bad, having been used on the three litre 911 Turbo and the first 928s before appearing on the 924 Turbo.
Simon