https://amazon.com/Sunway-Solar-Maintainer-Motorcycle-Powersports/dp/B01MYVUSRH/
Doesn't have to be this one. BUT BUT BUT
Make sure it has a diode built in to reduce/prevent overnight power loss. A 4 or 8 Watt panel is not going to overcharge the battery.
I use a 30 Watt solar panel with a real charge controller for my generator. This assures it has adequate battery for starting and prevents over-charging.