I think I'd be tempted to use some sort of low profile ring-type deck hardware, and install them with backing plates or washers. Possibly recessed type if you don't want anything protruding on the side. This would be strong and might tend to keep the hooks and strap off the platform (from rubbing on the sides, etc.), which seems like it might be an issue with just holes.
Schaefer makes some removable eyes that just leave a female threaded hole when you take them out (plus there is a plastic plug you can put in the hole if desired.
Deck hardware probably doesn't fit if you are trying to keep it super inexpensive though. It's possible there might be some application for VHB, but I'd be nervous about the generator falling off if it failed.
Another idea: Do you use your swim platform for walking over? If not, you might consider replacing a couple of the ladder fasteners with eye bolts and then fastening the generator to those. You could always put the regular machine screw back at a later date. (The top of the platform is cored, so you'd want to seal off the core, especially in any hole where you are removing/replacing fasteners; but other than that there is no harm in not bedding one of the fasteners since the other side (bottom) just leads to the water). I'm picturing the eye bolt on top of the platform. This would also tend to keep any hooks off the gelcoat, since it'd be on top plus there are the metal "top plates" for the ladder backing.