Sealed in really well? Hmmmm. I assume screwed in with some type of sealer? I have used a good stout steel putty knife to start things. Of course take out the screws first. You might also try a heat gun on low or even a hair dryer to try and soften the sealer. It's kind of hard to guess, but I would guess they have been in the sun and are probably brittle. No salvage value, just go slow and try hard not to damage the surrounding fiberglass. Depending on what kind of sealer was used, they may pop right out. The key is to measure the current mounting hole, not the over all speaker diameter. The manufacturers will give you specs for the required cutouts, believe it or not there is not a "standard" per se, although speakers are listed as a 5.25" the actual mounting holes can be a little different because of the profile of the mounting area. Second thing will be the screw holes, if you are very lucky the screw holes on the new speakers will line up, otherwise be prepared with a drill and bits. Lastly, the thing I always get burned with is the speaker leads. It seems like every manufacturer has their own idea what kind of connecters plug in to the speakers. I don't think I ever found a match so be prepared with electrical connecters, wire strippers and crimpers. Make sure you remove all the old sealer. Try scraping it off first, you may get away with using something like isopropyl alcohol to clean the surface before applying new sealer or you may end up needing acetone or lacquer thinner if the stuff is stubborn. For new sealer be sure to use something remains pliable after it cures.