There seems to be a lot of confusion, so for those who may still be unclear this is a summary of available small bike sprint racing classes (Endurance has options also).
(Sorry if it's hard to read, GTAM butchered my formatting).
There are 4 classes over 3 organizations, as follows:
SOAR:
Lightweight Production
Lightweight Superbike
RACE:
Lightweight Production
Lightweight Superbike
300 Production
CSBK National:
Kawi 300 Spec
Class details are as follows:
Kawi 300 Spec (CSBK):
Kawi 300's only, stock or kitted according to Kawi's whims (specific parts list). No full details available until next year - probably not the best place for noobs to try their hand by the looks of things as cost will likely be a large factor.
Lightweight Production (RACE, SOAR):
Class that includes _production_ bikes only. All allowed bikes are roughly in the same performance class, with the following displacement limits:
380cc Four stroke multi or twin (i.e. Ninja 300/250, R3)
400cc Four stroke single (i.e. RC390, CBR250/300)
500cc Four stroke air cooled (vintage legal)
400cc Two stroke air cooled street based
For those who don't know, the 2 stroke bikes allowed here are NOT GP machines, but are street based, which means that the power to weight ratios compared to, say a Ninja 300, will be about the same (the 4-strokes most likely have the advantage, actually). There are very few 2-strokes other than very old stuff that will fit this category. Every bike that could possibly be legal for this class will be in the same performance ballpark. This would be the class to race if you were interested in starting out on a small bike.
Lightweight Superbike (RACE, SOAR):
Class for lightweight bikes of increased performance but not up to 600 class standards. Much more variety is allowed here, with the following displacement limits:
500cc Four stroke, liquid cooled multis (i.e. FZR 400, EX500, CB500)
650cc Four stroke, liquid cooled twins (i.e. SV650, Hawk GT650)
Unlimited singles
750cc Four stroke, air cooled, multis (Vintage eligible)
800cc Four stroke, air cooled, twins (Vintage eligible)
500cc Street based two stroke
250cc GP based two stroke (Lost Era eligible)
125cc GP based two stroke
Lots of bikes could potentially be competitive here, but this is not quite the right place for a Ninja300 (even though it could be somewhat competitive with a good rider) as it is a superbike class, not a production class. Could be good as a secondary class just to get laps in. This class has the potential to be completely dominated by 650 twins and become Lightweight Twins II, but thus far that has not occurred.
300 Production (RACE):
This class was created quietly at the urging of Bakaboy to give 300 class production bikes a second racing class to be competitive in. Rules are slightly more restricted than Lightweight Production, but this is a fantastic second class for small production bikes at RACE. Due to the nature of how and when the class was created there is no information in the rulebook regarding its existence (bad on RACE).
All the above information is available at the respective racing organizations websites, or here at GTAM.
If you are interested in racing a 300 class machine, then there are options - you just have to come out and try it. You will be gridded with only machines of like type (excluding Superbike, but it is not a production class), and I bet you'll like it.