Bob your interest in the Villier's/Simplex combination is an entertaining prospect & one I suspect would have perhaps caught the interest of Simplex at some point in time. One drawback would have been the "liability" concerns raised with enhanced performance & the resulting speeds projected. Legal problems have long been a source of concern in the auto/cycle industries. Upgrading the engine & drivetrain significantly would have prompted a hard look at the frame, fork & rolling components as well.
What hobbyist have done and continue to do with power train, frame and components modifications and replacement are a far cry from large scale production of reasonably safe and reliable transportation; properly priced and attractively marketed...profits maintained as well.
Personally I'd hoped such a Simplex would be presented with photographic evidence & appropriate documentation as a factory prototype, on the order of the Harley "single cylinder" Knucklehead engine, but not yet it seems.
I've found the Simplex frame a difficult one to accept a larger engine/transmission combination with; while also keeping the beautiful tank, frame lines and proportions intact. The bike's stance & size is so important to it's beloved appeal that I feature the company would have been extremely reluctant to change it much if any to accept a more promising power train. My own extensive modifications to the 1957 Servi-Cycles dimensions and replacement of major component groups was the result of won't fit and don't have, while searching for more power and performance and along the way I added some more strength as well to accommodate the upcoming addition of a sidecar. I don't think Simplex was willing to go to the expense of adding a new model and producing two separate cycles either. My opinion.
Rick C.