I suggest a 500cc Manx Norton ridden properly, would be enough to scare the shit out of anyone on the IOM. When I watch those on-board videos, I often look at the ends of the straights and wonder which side of the road the bike should be on. You would not need to forget. My mate raced a Manx there in the 60s. The first time he went there he did an 84 MPH lap. But he got faster. Bill Horsman won the Junior Classic TT there, but he was an exceptional rider. He only rode there once. I think it was with an Aermacchi 350.
I think dragging a 3 cylinder BSA around there might be bad.
I would not ride there, I have the wrong mentality for that. I would be certain to over-cook a corner somewhere.