The safe recommendation would be to use the master from whatever GSXR you got the calipers from.
If you're thinking radial:
At 32/34 (stock gixer four pot piston sizes) you're right up in 19mm radial territory so I'd be looking at the Nissin unit (as found on zx6r/zx14/cbr600rr and I think maybe even some of the gixers).
There's an 18mm Brembo radial used on a couple of triumphs but I'd only consider that if you prefer a softer lever with a fair bit of travel.
Don't be tempted by the R1 16mm Brembo affair. It'll not be enough m/c for your calipers - it barely handled my H model ZXR calipers with their 30mm pistons(was nice with the original L calipers - nice feel anyway - didn't fix the pads being too small).
Do be tempted by a Brembo RCS19.

The euro is weak at the moment, I picked up one a few weeks ago from Germany for 2/3 of the UK RRP(postage included). With my calipers the 18 setting is lovely and the 20 a little wooden. Your pistons are the same size as Brembo M50s so I'd be willing to bet it'd be a sweeter match and you'd be able to choose between the settings.
Even from Germany or Italy you're still looking at twice the cost of a second hand nissin for an RCS. Whether it's overkill for a roadbike is debatable but I have no regrets and don't imagine I ever will.
Mind if you fit any radial m/c you'll have to whip off the fairing & clocks and give the clock bracket a few gentle whacks with a mash hammer until the brake lever doesn't hit the bottom of the rev counter at full lock. Tilts the clocks forward a fair bit but they're still readable.