Just intuitively, I'd say that definitely not with a P-20 sized pipe and most likely not with a K45 sized pipe.

These 70N engines produce almost twice the thrust of a 40N engine but with the same can and nozzle size. To do this, they have to move more air through the same opening - thus much higher efflux velocity. I would think that stuffing that much high velocity air (or trying to) through a small pipe would at a minimum result in a loss of thrust, possibly engine overheat, or worst case pipe collapse.

I would stick to a pipe designed for an 80N engine for the K-70 myself...

Bob