I'd suggest trawling about on Jockey Journal for a bike like that. One here in a stock Triumph rigid frame http://www.jockeyjournal.com/forum/show ... umph+frame
which interestingly enough has the engine and trans in the stock locations; poster says it will fit, and there's the photo.
You could also use the front section of an earlier swing-arm frame, the results would be similar in terms of geometry and the appearance would be better.
"Back in the day", bikes using 45 engines and British frames and transmissions were built in some numbers in the UK, because the components were available. The most common approach was the cut off the top tube in front of the seat post and fabricate a new front section, keeping the various forgings etc for transmission and oil tank mounts. Also, British frames tended to have more ground clearance than American ones so the "British" style of chopper tended to be less concerned with raising the front of the frame, although there were plenty of real horrors about.
Another approach was to take the widely-available BSA M or B series frame and extend it by cutting and extending the single top-tube, along with the lower frame rails if fitted ( the M33 and M21 have them, the M20 may or may not, the B31 and B33 don't - but any rear section fits any front section ).
Any British frame conversion leaves you with the task of fabricating the engine mounts, you really need a frame with full-length bottom rails to accomodate this - the 45 engine isn't designed to be used as a structural frame member, unlike the Indian Scout/741 unit.
Interesting to see another bike so similar to my project on this thread. I came to the conclusion many years ago, after owning a bike of that sort, that the Ariel was much the best suited of the period rigid frames for the purpose, although always difficult to find and tending to vary in detail between successive batches - the downtube length varies, for example; the forks have several different configurations, but they are all more-or-less interchangeable as complete sub-assemblies.