You can use a Group 51R battery (not 151R) with a slight modification. The 51R battery has much higher CCA.
There’s a plastic battery tray underneath the stock battery. The 51R battery is about an inch longer than the 151R, so you have to remove the plastic battery tray and remove the lip or side of it. The 51R battery will “butt” up against the air filter box… but other than that it fits perfectly.
Good luck.