My Bottpower XR1 has a Super-B battery on it which is a 7.5Ah
The Buell engine needs a fair bit of umph to get it to crank over and it turns over ok but not what I would call a spirited manner.
The battery is very compact and incredibly light. Stupidly light, in fact.
I use a NOCO Genius2 charger (2Ah) to keep it topped up.
You could perhaps consider the same 7.5 Ah or if you have space for it, go for the 10Ah one.
Here's a link:
Super B Andrena 12V10Ah Lithium Battery