I think Dave T would be on the money re performance at 40 to 45 mph.
I'm not sure of the 31, but my 27 frame is pretty light. An equivalent light sidecar would be the best option.
The early front brake is not good. I have 'vivid' memories of the bike loaded with gear coming to a roundabout and me making eye contact with an oncoming car, signalling I could not stop.
You might consider sacrificing some originality to adapt a later cast front brake or similar.
As Nev points out, sidecars were once a universal transport. For a passenger, they are warmer, cut out a fair bit of wind and wet and better for conversation than a pillion seat. They are also better for carrying beer, luggage and tools.
Make sure someone is filming when you go on your first ride..
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.