If you play with PC Speaker sound effects and music, Monkey 1 and 2 always work fine on my Pentium MMX @ 266 MHz, so they shouldn't have any trouble on a 486 @ 50 MHz. Just run the game with the parameter "i" and you're done (enter "monkey i" or "monkey2 i" at the command prompt). I know that Monkey 1 might have issues on fast systems when using AdLib/SoundBlaster audio, but haven't had any issues with Monkey 2 so far. I hacked the Monkey 1 executable myself and this fixed the divide by 0 error on my system, but you I guess you might want to use the official 486 patch for the game instead.
When running the game, you might want to force it into Adlib/SoundBlaster mode (by entering "monkey a" at the command prompt), otherwise the program might not work correctly.
Edit: I'm talking about the VGA floppy version of Monkey 1 here. I have no experience with the EGA or CD versions.