3M makes adhesive removers, that may work. I haven't tried it on your particular application but may be worth looking. They offer various types, including a citrus based aerosol one that seems to get good reviews. I used something similar when I was removing 50+ year old undercoating recently with good results (as well as wire wheels and abrasive fiber discs on an angle grinder).
Google 3M adhesive remover, that should steer you in a good direction.
For general adhesive removal around the shop I'll usually use brake cleaner, or DuPont pre-cleaner for body work (basically a Prep-Sol type of product). In your case, with all that dried on glue, those would require you always keep it wet to loosen it up. I tried those with the undercoating with fair results, but was pleasantly surprised that the citrus aerosol cleaner worked good, and stayed wet without constantly requiring re-application. Either way, it's sure to be a crappy job.