Not a car built by John Griffiths !
John A. Griffiths was the owner and builder of J.A.G. cars. You have me confused
Anyway - you say it's not Ford V8 or 10hp engined, nor MG, Coventry-Climax or Riley engined. I cannot find any reference to JAG cars which were fitted with anything else.
I therefore assume that this particular car has been re-engined at a later date, and that means almost anything could be under the bonnet. Is it a Mercury (aka Ford) engine?