I am not telling you to go down this road, but...
I guess you could make any relay rated for enough amps and continues duty work. It does seem a bit high priced for a simple relay. All it does is connect the generator to the battery when the go peddle is pressed. You could check the surplus supply type places (surplus center, etc.) Just make sure that the Contact Rating is high enough and the Duty Cycle is 100% Many of these industrial supply places provide all the technical info you would need to determine the amps it can handle and the duty cycle of the stuff they sell.
But then, you would have a cobbled up cart. Unless you really can't afford the right parts, you really should not try to make due with something that just might work, get the right parts for your cart. Just like using the right tool for the job, in the end, it's much easier.
