Where (what country) do you live?
In my country, even relatively simple and safe LPG conversion is illegal to do-it-yourself. You are required to have it made in certified service center and you are subject to annual inspection. The LPG conversion itself costs about 1200 Euros, CNG conversion - which is closer to what you propose as you intend to use high pressure tank and valves etc. - is about 2500 Euros and legislative is even more strict, given 250 bar pressure and high flammability. So idea of "everyone can do it in his backyard" sounds near impossible to me, at least in the E.U.
On the other hand, you can buy inexpensive hubmotors and build something like
Podride - with 11 kilometers range it is on the same level as hydrogene converted car - and wihout any government made obstructions.
-- Also, you face danger of explosion already in the process of making the gas, whether your customer has tanks on roof or not. I dare to say that cars with hydrogen tanks would be actually much safer than your gacility to bubble and compress it. HHO mixture can actually explode without any spark thanks to heating during compression.