thanks for all the ideas. Got a bit to think about and options to try. My plan was to by my son a bigger pit bike or a Jap bike - TTR125 or similar. But a bit more confident now i can come up with a solution.
All good mate, def get some pics or keep in touch with your ideas, im interested in the project and will be a great option for others in a similar situation.
One from way left field, what about a thumb clutch? Doohan used to have a thumb brake on a couple of his bikes? If you fitted a four wheel style setup (with the thumb normally as an accelerator) and used the accelerator lever as the clutch? Could be a cheap fix? Front brake lever thumb throttle 50 110cc 125cc 150 250cc atv quad buggy bike | eBay