I think I lot of Interstates ended up with peashooters because in the late 70s it was sometimes hard to source parts. I know I struggled to find Interstate mufflers in that period. Of course shopping was done via the phone book and the yellow pages had a void under "Norton" and I was young and didn't have much experience with, well truth be told, anything! My '74 Interstate did have Smith clocks though. And it ended up with peashooters and upswept pipes because its what I could get.
These days I hardly worry about whether my bike is proper or not. It's sort of like my face. When I'm using it, I don't look at it much! Which is a handy thing, considering my face.
I think the roadster in question in this thread has some very desirable things, such as color and seat, I'd be looking to lose the handle bars and the rear wheel, and would make any offer appropriate to having to swap those things out. That's just me, but I gotta think that most vintage Brit bike shoppers are thinking cafe bike as opposed to semi-chopper.