I would avoid the lithium battery if the bike has the stock regulator.
The mechanical regulator isn't very compatible with lithium and its voltage requirements.
Oh it'll charge enough but it allows voltage spikes through that while a lead acid will shrug off, a lithium can be damaged by, some have even started on fire.
Now, if he has switched to a solid state, then he would be ok. He would just need to balance the battery every so often if it isn't capable of doing its own balancing.
Now, I do know a small 7 amp lead acid agm will work perfectly well, even with electric start as long as the bike starts good. Just don't get stuck in traffic a long time.
In the manual section, there is an article about magnetos that may be interesting. And may answer other questions you didn't know you had.
The ride IS the adventure. The destination is just to get gas!