I don't think you want to be sticking an electronic component with an LCD screen into an oven..even at a low tempature.
Headlight housing is essentually just an empty plastic shell where the glue melts long beofre the plastic does.
I would try the acetone trick first, but I don't think it will work. You could try more directed heat using a heat gun and gently pry it apart, but then you risk warping the plastic which will make it next to impossible to re-seal it. As a last resort, cut it open very carfully using Dremel rotary tool or a small hack or coping saw.
Might want to price out how much a replacement spedo unit goes for in case the worst happens.
Heck, it might be worth an email to Yamaha asking for their input or any info on if it can come apart?
