I had one of those about 5 yrs ago. I LOVED IT!!
ZERO Bottom end...and almost nothing will help it.
I tried pipes, reeds, everything.
I didn't mind it though, I always loved racing 125's and revving their guts out.
Good thing for you is, you WILL HAVE TO LEARN to keep it revving and on the pipe. If you let it fall off, it falls on it's face.
Once you learn to ride that thing pinned and on the pipe, and you move on to another 125, you'll ride it like a pro. The 1982 RM125 is a great learning tool, if you want to learn how to ride on the pipe.
It's a lightswitch. It's on and screaming or it's off and bogging to get out of it's own way.
Rev it until you think it's going to blow up, then shift.
Keep it pinned brother, that's the only way you'll get it to go.