Barry,
Is it an 18M or a 38M? Either way, its worth having.
18M was 4sp w right hand gearchange - bottom end v similar to 4sp 250 engines from the 60's. But it was discontinued in 73. Not real competitive in pre75 125, tho Ed Lancaster apparently won an OZ title on one in 72 or 73. Had fragile bigend end, but as stated above, Lyall Obrien replaced b'end brg w an off the shelf INA caged roller (fit straight in std rod & pin apparently) & had v good reliability.
38M was 6sp, crinkle-fin & left foot gearchange, but didn't come out as prod'n model til 75-76. Factory models were around in 74 tho. If its one of them, its rare as. Much more modern engine design, but still had fragile b'end. However I suspect they would also take an INA caged roller. (I put one in myV75 Cappra too)
Either way, I'd grab it