You can always go with a Borg Warner electric overdrive behind a 3 speed. I cruise 60 mph +/- at 2000 rpm with mine. Should be a straight swap with what you have now, no drive shaft changes either.
'59 Apache ‘21 Beta 390RS ‘18 BMW R1200RT '12 Ford Flex '13 Aprilia Tuono ‘11 Ducati Hypermotard Salisbury, MD