[Dean]

3.3.3.B.15: Any wet-cell battery moved from the manufacturer’s original
location shall be in a non-conductive marine-type container
or equivalent, and the hot terminal shall be insulated. All
batteries (on-board power supplies) shall be attached securely
to the frame or chassis structure independent of the
marine-type container. NOTE: This will allow the use of gel
cell or dry cell (AGM) batteries without a non-conductive
marine-type container where applicable.

Stock: No changes allowed
Street Touring: 14.9
The make, model number, and size of the battery may be changed
but not its voltage. Relocation of the battery or batteries is
permitted but not into the passenger compartment. If the battery is
relocated and the original battery tray can be removed by simply
unbolting it, the tray may be removed, or relocated with the battery.
Holes may be drilled for mounting or passage of cables. Longer
cables may be substituted to permit relocation. The number of
battery or batteries may not be changed from stock. The area
behind the rearmost seat is not considered to be within the
passenger compartment.

Street Prepared: 15.9
B.The make, model number, and size of the battery may be
changed but not its voltage.
C. Relocation of the battery or batteries is permitted but not into
the passenger compartment. If the battery is relocated and the
original battery tray can be removed by simply unbolting it, the
tray may be removed, or relocated with the battery. Holes may
be drilled for mounting or passage of cables. Longer cables may
be substituted to permit relocation. The number of battery or
batteries may not be changed from stock. The area behind the
rearmost seat is not considered to be within the passenger compartment.
The area under the rearmost seat is considered to be
within the passenger compartment.

Beyond that, I think it is open, but you must comply with 3.3.3...