Get a 6V Cyclon pack.
Cyclon sell individual 2V cells in a big variety of capacities.
Paul Groff sells a standard 3 cell pack which will fit into your battery box.
Cyclons are pressure valve regulated, spiral wound lead tin batteries that will go for up to 3 years without self discharging.
Failing that get a Pressure Valve Regulated, Absorbent Glass Matt, sealed 6V battery.
Similar story, totally maintaince free, can be fitted at any angle including upside down, capable of being charged at 10A or higher without damage and capable of massive discharge currents.
Avoid gel cells as a direct short or very high discharge or charge rate will stuff the battery.
Also avoid any battery with open cells they will be designed for modern sewing machines and fall apart the first time you go for a ride.