I think a narrower chain will work, the chain doesn't engage with the smooth pulley wheels, so it shouldn't bind on those, the pulley plates will be a little further apart than a 3/32 RD but it'll probably work. It probably wouldn't the other way around.
I have a 4 speed Simplex 51 (model after TdF) - but that is marked 2.38 -so not a 1/8" chain - but I think they still made them in both sizes.
I think a narrower chain will work, the chain doesn't engage with the smooth pulley wheels, so it shouldn't bind on those, the pulley plates will be a little further apart than a 3/32 RD but it'll probably work. It probably wouldn't the other way around.
I have a 4 speed Simplex 51 (model after TdF) - but that is marked 2.38 -so not a 1/8" chain - but I think they still made them in both sizes.