Hi,

I'm almost sure it s a HP 61 F-RC with its orginal muffler. See image below of such engine from 1973.
They was made in end of 1960th and beginning 1970th.
I know a guy in my RC club in 1970th that had such engine, he used it ina Midwest Mach 1, it was 1974-1975.
Good power but rather high soundlevel, nice blue cylinder.
Muffler shape, top (4 screws) and carburator throat looks same on your picture and the picture I posted here.

/Bo