Torque not Revs
you want a motor that is Torque monster not a high revving HP beast!!!!
The BMW airhead twins before 1979 or 1980 had a heavier flywheel which gave better Torque. Twins like the Yamaha XS650 give goo Torque comparable to the Triumphs but yamaha reliable. I think a Honda CB750 4cylinder would struggle to get a sidecar moving slipping clutch higher RPM but it has been done.
the 1200 CC Suzuki Bandit is a good tug