Based on a bad experience with the S&W 622 (.22LR pistol with aluminum barrel shroud) where the scope mount screws stripped the threads a number of times, I wouldn't seriously think of milling/drilling and tapping a 642 to put anything on it! You might also weaken the top-strap with potentially catastrophic results.
Just recognize the 442/642 for what they are . . . they're belly guns. If you're shooting something more than 10-20' away with these guns, you are using the wrong gun!!
CT laser sights are the best bet for aftermarket sighting solutions on this gun, plus the rubber grips help absorb some of the recoil.