25 or even 26mm would be no problem. I have a 110 and mounted a 26mm which started right up. Difference was that the engine no longer struggles as much in high revs like it pitifully used to.
The only problem I had was that the carb could not be fitted as the bottom part touched the cylinder fins. Instead of shaving some of the fins off, I cut, twisted and welded the manifold that the carb now faces front left if you're seated on the bike. I managed to use the original manifold but after match porting it, I reckon I was a merely a few mm's left but still managed to keep it leakproof on both the head and carb side mounts.
As to carb brands, I have yet to see any difference between the Mikuni's and the China copy's (Keihin, Zhengshen, or whatever). As long as it physically looks the same, I can settle for the cheaper one. I've seen at least 3 copies so far (couldnt remember the brand of the 3rd one) and are being used by me and my friends with no issues reported whatsoever. They all look the same.