Not too sure on the carb size as I have not had a good look at mine yet, but am fairly sure it is the same as the GPR50, namely 17.5mm. I have been unable to find any engine specs. specific to the GPR75, (if you find any I would appreciate a copy), only for a GPR50 with a 75cc kit.
Metrakit who make such a kit sell a 21.5mm carb to complete the conversion, so 22 mm would be about right.
I recently talked to Oz at Cycle Imports who seemed very knowledgable on the GPR and he mentioned that he was looking into producing an adaptor plate/manifold so that a bolt on carb could be fitted the size he mentioned was I think 22 mm, but could have been 24mm, not too sure!.
Anyhow you might want to call him and have a chat, he also supplies other tuning parts, tel # is - 1-305-597-4440, web site, www.cycleimports.com. Hope this helps.