I used to have a small scooter and I remember the lock being more of a formality than security. Not something I would want to leave a firearm in. Do what you can to beef up the lock. In addition see if you can figure out a way to put a Kypton cable with a real lock on it. You can then secure the cable and firearm directly to the frame. It may take some drilling or cutting to gain access to the underside and frame but it will deter the guy with a screwdriver that pries your compartment open.
I did such in my truck as the console doesn't lock (not that the oem lock is worth anything). I used an eyebolt with the cable. Eyebolt is bolted through the floorpan and cotterpinned on the underside. Cable goes through the eye, through the trigger guard (after safety unloading and clearing...and rechecking), then a sturdy lock is attached.
You will have to be creative to disarm, unload, and clear the weapon out in the open. Maybe there is an out-of-the-way place where you can discreetly take care of business.
It's not 100% but it will get through a smash and grab. Bolt cutters would even have a harder time since it is a tight compartment. I don't leave it locked up for extended periods or overnight.
Just the thought of leaving a firearm on a motorbike is a tad scary to me. Be careful and think it out.
Kyptonite cable for bike seats. I know it can hold up to some attacks as I have a mauled one that someone thought they could cut through to get a tool box. I still have the toolbox and cable.
http://www.kryptonitelock.com/Products/ ... 1&pid=1122