bsatom's bike seems to bear out panic's assertion, that you can do it with a BT flathead if that's what you really want to do, but quite what you gain by it is a good question. Photos certainly indicate that an ohv engine will not fit without major surgery. There was a thread about the converse, a 45 engine in a BT frame, and the consensus appeared to be that the BT is a superior frame in all respects and not significantly heavier.
I'd suspect that the 45 transmission ratios would not suit the larger motor, quite apart from alignment issues, and the ability of the mounting and cross-over drive pattern to cope with the increased torque would have to be suspect ( best configuration is the primary and secondary drive on same side with all 3 hubs in a straight line, as per raised gearboxes on BTs as a racing mod )
given that very few custom bikes do any serious mileage or real work, you could probably get away with pretty much anything if that's what you really wanted to do... but it would be a shame to cut up increasingly rare and expensive parts for that sort of reason IMHO
Shoot, a man could have a good weekend in Dallas with all that stuff...