If these larger kits exist, the extra expense of needing a new crankshaft to cope with extra torque would make the cost higher than the 300 kit.
This depends of course on how tired the original is but I am building a 300 and I am using a new crankshaft. The old one will be perfectly ok in one of my standard engines but I wouldn't risk it in a 'very radical' 300cc.
If you look through the forum, you can read how some guys do not like the 300 due to the loss of 'kick in pants'. This can be overcome in the 300 by using lighter pistons; but would be very difficult if you went any larger.
70 something horsepower is enough or you might as well buy a 4 stroke.
I have been promised 80+ from my 300cc