Im not sure what is 'lemony' about the rs125 motor.
its a 15 year old design, goes 10,000km between top end rebuilds and 20,000km between bottom end rebuilds.
With simple porting, pipe and carb you can bring the bike up to and beyond what a modern 125MX engine can do.
But yes I agree, the optimum thing to do is swap out the rotax 123/124 and then insert a rotax 128 disc valve motor (no frame mods) or 257 motor (minor frame mods).