I don't really see the point in swapping engines. The work involved wouldn't be too hard, but I suspect a 125cc would still be classed as having the larger engine by the DVLA unless you went through all the hassle of re-registering it as a 50cc. You'd then have to do the same again when putting the 125cc engine back in when you turn 17 and gain an A1 licence. So, it would be legal to make the engine swap, but (as well as the hassle of the paperwork) insurance could also be a problem, especially if the bike is still registered as a 125cc machine that would be illegal for a 16-year-old to ride on the road. It's far better to buy a cheap 50cc bike and wait until you're 17 to get a 125cc supermoto. Insurance is going to be pricey on a supermoto either way as they are among the most stolen bikes, so probably best to wait a year to gain some no-claims on cheap 50cc bike.