...and when I pulled the head it had very little carbon deposits inside and around the cylinder bore/head/exhaust, so I think we can rule out a worn clutch
Hello TRV
I guess you are stuck with a fast machine then.

I feel there would be very little carbon deposits though.
The Imperial had a cylinder head spacer and ran BP Zoom Outboard (50:1 mix).
I believe you would have read my History Record on the Imperial.
I argued there that historical baggage gave the Imperial a high landroll speed.
The beauty of the Scott Bonnar Model 45 was that the designers thought
outside that baggage - they used about the same reduction ratios but
incorporated a clever 'hair-trigger' clutch - that reversed the operation
of the landroll clutch. The new approach permitted variable roller speed to
suit the operator. Just Brilliant!
Nonetheless, the Imperial
does have its charms alongside its idiosyncrasies.
For the record, I would love to see some images of your machine.
Cheers---------------
Jack