As far as the distortion, remember these were made when fifty wpc was considered high power, and that they are a small speaker. They aren't really made for cranking. That isn't to say they won't play fairly loudly, I used to be able to hear my mini system from the living room when I was out on the porch.
The stickiness is from a coating that seals and protects the surrounds, it's good that it is still there.
Reusing those cabinets isn't too bad an idea, because you probably couldn't build new cabinets for anywhere close to what you would be likely to get if you sold the MC 500s. HTHMAN is correct that replacing drivers willy-nilly will likely yield a worse sound than what you had when you started.
If I were planning to use these with different drivers, I would pick a woofer whose needs matched the air space in that box, and then pick tweeters and crossovers to match the woofer. I wouldn't bother trying to match the physical dimensions of the old drivers, but would replace the front baffles with baffles cut to match the new drivers.