A real good exhaust is were I would be looking
There are at least 3 different types on FF's thread.
If you look at them several follow the style like the original looper had.
Seems to be the go as used on the M31 as well.
I reckon you could make that yourself
Guessing but I would say the reason the looper went to the style of exhaust like a 70/75 has is to broaden the torque curve, not something you need on a kilo engine.
Those were the days of cross flow grunt and the loopers had less. Probably explains the low gear ratios as well.

PM philmyrna_shakesby he knows a thing or 2 about loopers