If it's performance you're after- (i.e. spread of power rather than just a raw h/p figure), then your idea of ditching the disc and replacing it with a reed block is sensible. The disc system is capable of producing great power, but it's always a compromise and a disc that produces good power up top can stumble and have blowback down low. Not as bad as a piston port engine with it's symmetric inlet timing, but still not ideal. The reed on the other hand guarantees no blowback and will work whereever in the revs that the pipe, transfers and exh port are telling it to.
You are right, there are plenty of high performance kart and outboard motors that use this system. And indeed the crankcase reed bike motors that I think most racing bikes use now have no piston control of the inlet- it's all down to the reed.
My brother did a disc-to-reed conversion to a Suzuki GP100 in a bucket racer about 10 years ago, and the engineering (i.e. alloy welding/machining) was a bit of a mission ( but nothing that a clever bloke like Loz wouldn't do before breakfast most saturdays
) Because my brother is a greedy fella, he used a massive six-petal reed block from a YZ490 or something and it was much too big. The engine produced magnificent bottom end, but nothing up top. I suspect that the huge reed block had the effect of very substantially increasing his crankcase volume and stifling the transfer pumping at high revs. Actually that's just a guess. It wasn't a success anyway and he re-converted it back to a disc with another set of cases. But the idea was sound, even if the execution was suss.
Incidentally the GP100 cases welded up for the reed conversion are somewhere in my shed if anyone wants them to have a play around.
If you do stick with the disc (and your Big Horn could be made into a real monster with some porting, disc and pipe work) have a read of the Bell Two-Stroke Tuning book, but don't forget the relationship between the disc opening/closing and the transfer opening position. If you cut the disc and don't time it to the transfers, you'll make it peaky and hard to ride without a c/r gearbox.
Have fun !!
Nitram