Peak torque

I think you may have to be content with the sort of statement  that Pa Norton would have made. "Power is sufficient".     I would guess about 40 HP.   Wait till you get it wound up to 6500 at full throttle  in 2nd  ,  WHEEEEE !!.

You could always try sticking it on a rolling-road dyno.

Torque (in lb.ft) = (5250 x HP) / rpm.  

With an efficient engine, torque curve should  be fairly flat over the usable range.  So Bhp should go up in nearly a straight line with rpm  again over usable range.  So if the book says 40bhp at 6000 rpm (or something similar), the formula can give you the torque at that rpm. And assume the rest follows linearly. 

Torque and therefore power will fall off at the top end.

I wonder how accurate the dynamometers were in the 1960s anyway. I remember using one in our engineering labs and my results were less than convincing   even to me...

I guess the 'flatish torque curve' only applies where the engine can breath effectively. Perhaps between 2,000 and 5,000 RPM for our older style engines?

The apparent torque from a Commando at 1,500 - 2,000 RPM seems more to me than a new BMW 1200 at the same revs, and that has twice the peak BHP. That low down Commando grunt is one of the most appealing aspects of the bike.

The ultimate low rev torque comes from a steam engine that will develop its greatest torque at zero RPM!