I have my order in for the Stark also, and I will be needing a 240V power source to charge it at the track. Here is a Inverter Generator that puts out 21 Amps at 240V, and only weighs 86lbs. I bought one of there other generators, the MXR3500, only 120VAC, and it has worked out really well for my application. I run 2 15Amp 72V chargers on my DIY Electric YZ 250F Yamaha. I split the battery into 2 packs so I could charge faster, 15 Amps each on 120VAC.
There are 2 models weight versus cost.
114lb model Portable Inverter Generator 5500W Max 5000 Watt Gas Electric Start w/ CO Alert | eBay $999
86lb model Portable Gas Generator 5500 Watt 120V/240V Electric Start w/CO Alert 5KW Running | eBay $1168 It is the same manufacturer, but I haven't bought from this vendor
The one I bought MXR3500 120V 47lbs
MXR3500 3500 Watt Portable Travel Inverter Generator Gasoline Super Quiet | eBay $575