Everything in the 4K motor is small, but the carby is a good place to start. If you can't replace it, you can rejet it. I increased the primary jet from about 1.21mm to about 1.3mm, and the secondary to about 1.5mm. It goes better now. I also removed the choke and extra butterfly in the secondary barrel, which improves throtle responce and flow. The small exhaust manifold (and everything else) are still holding it back, as well as the inablility to rev hard due to the pushrod design with weak springs. The carby stud pattern is the same as used on other Jap engines of that time, such as Datsun 200B's and maybe Coronas. A poor mans option may be to fit a larger carby off a 200B that is jetted to suit (if you can't afford a webber or something), and you may even get wheelspin as I did! For a bit of rpm you really need to get some fresh or stiffer valve springs.
It's a long time ago now that I had a 4K engine. Mine was not very good because I spent all my time building the 2T-G. For handling the 4K is better because it is lighter than a 2T-G. The 2T-G makes my car a bit heavy overall, and front heavy also.