Check with Chris Jones as he is the MA representative for minimoto. The engine capacity limits are set and there is no provision for a 5% or even a 1% over capacity allowance. I can't remember off the top of my head what rule number but it is stated in the manual of motorcycle sport issed by MA each year.
I think you will find that the yx 160 is allowed as is is maketed as a 160cc engine, how ever be careful with a 143 bbk as it is known exactly what the capacity is.
I personally would be sticking to a Zongshen 140 modified with a head and piston. It won the nationals, and I would go as far to say that it would be more reliable than a big bore kit on a 125.