Not seen an impco dual fuel set up, the dual fuel set ups that I have seen seems to be by Prinz don't know if this is down to the fuel injected engines now or just different system.
I had an impco system on my old 190 with the ford 351 but just on LPG.
We have a guy in our club knows and carries out LPG installs car and boats if I see him I will ask.
88 190, as for the custom removable tank, over here in the uk some of the boats running on propane use a fork truck bottle instead of fixed tank, just swap gas bottles and get the bottles refilled, although seems a pita to be swapping bottles I guess it works for some.
More common these days for lake / club to have gas tank on site so easy filling.