The classic 'bikers need three or four arms' modeling dilemma
I've seen conversions where the grav gun (trimmed down to reduce length) replaces the bike headlight, which can look pretty good.
The obvious 2 options for the shield are:
- an upper arm mounting, allowing the hand to grip handlebars
- modify the bike front fairing bit to look like a giant storm shield (retaining the bolters but probably covering the headlight). Fluffwise, this represents a bike-specific version of the normal shield.