Yo Karl_ might not be to late ...
"This 1955 Nimbus Type C is powered by a 746cc SOHC inline-four mated to a three-speed transmission and is finished in olive green.
The Nimbus C might definitely scratch that Henderson/Indian itch, although I think one would notice the difference between the ~20 hp it gives and the ~30 to 40 that the Indian 4s put out. The Nimbus is quite a bit more financially attainable however. We have quite a few that show up at local moto events, members of the Chicago Nimbus Club.
https://www.nimbusclub.com/ I like the open valve gear on top of the engine!
I was thinking I might satisfy my personal longitudinal inline desires with a Sunbeam S7 or S8.
I haven't seen one locally, but they don't seem to be uncommon or particularly expensive in the UK.
For Viczena, there's always the Chang Jiang CJ750 M1.
I saw a CJ750 offered up locally, complete with sidecar, for less than $6K, although this was the OHV version.