How hard is it? I just got 4 A500s (one of them in great condition, but with 1.2 and the rest in really bad shape, but working and on Kickstart 1.3)
http://www.freemansgarage.com/stuff/P1010006.JPG
You could just swap the motherboard
The tin cover is not for dissipation of heat from what I recall. I believe its purpose is to block the RF noise the motherboard produces which can interfere with old school TV reception. With digital TV and cable, I doubt running without the RF shield will pose any problems.
I'm not an electrical engineer (I don't even play one on TV) and could be wrong, so if an airplane lands on your house, your wife can't watch Oprah or the FCC shows up at your door...
