I'm going to sound really ignorant here, though do you need a battery start bike for a GPS set up?

No you don't !
I had a KTM 350cc EXG 2-stroke kick-start from 1989 it had the wiring for a small 12v/3A battery to get the lights working if the engine was off and I used it to run my GPS off it as well.

make sure the battery is behind the regulator to save it from peaks, only needs charging nothing else.

My mate fitted the same battery to his Honda XR250 with kick-start only for the same reason to run the GPS and it works great.

o.k. thanks.

This guy has done something similar/ he also posts on here username wonkey
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well I forgot to say that I used one of this sealed 12v/3A gel-batteries, that type small energy blocks used in power toys about 4x2x2 inch fit's easy in to the bottom of the air box..... I did pick up a few of this batteries on the "Old-Sodbury Land Rover sort out" for £5 quit each.

But you may get this type in Caravan shops as well this batteries are used for water pumps, or have a look around in shops where they sale EL-car models with remote control.....