I wasn't aware that you had a gas engine already (I made my money on the hull and rigging, not engines
). I think Greenstar was pretty reasonable about his proposal, though I am a little hesitant about his battery usage profile. It isn't that I am opposed to swapping to electric, just that most people don't have reasonable expectations about what electric can provide.
Further I have questions about how they can make the assumption that a 5.8kw continuious engine has the same rated output as a 18hp diesel. HP (or KW) is the same no matter what the source.
Finally, adding a generator... It is certainly doable, but using a gass generator is going to be somewhat cumbersome. Most marine generators are diesel, and the couple of portable generators (like the Honda 2000) that are gass, really are not suitable for permanent installation. I have seen it done, but still question the real risk of fire and explosions from this type of installation.
Frankly it comes down to what you are trying to achieve. If as you mentioned before you need a few hours of run time each way then I highly doubt it as a suitable option. If you are ok with just a few minutes then it is certainly workable.