someone else had a question about J series engines a while ago too - maybe it was on 1200.com... i thought they were the same but apparently there are some serious differences. here are some things i learned
the J series is a metric version of the BMC A series.
there were some earlier versions, but the most common are the J13 and J15.
the J15 has a longer stroke than the A15 - and no, the rods and crank aren't compatible - the rods have a different journal size and i
think i read something about the crank journals being at different centres too.
the heads have about the same flow as an early A12A head. i dunno if this is the same as any other A12 head though...
an A series is almost a straight bolt in - just use the J series mounts like you would putting a 120Y A12 into a datto 1200.
i don't think anyone has built a "performance" J series because they rev like a diesel due to their LONG stroke - but for a peak hour crawler i dare say a little weber and a nice cam and it would be a very economical daily.
that's about it from me...