Microlite20 "Golden edition" has rules for natural healing as well:
Characters heal naturally at a rate of 1HP and 1STR per day of rest, 3HP and STR
bonus (minimum of 1 if no STR bonus) if complete bed rest (doing nothing for an
entire day). For each complete week of bed rest, the character can add his STR. In
both cases above, the character must have adequate food, water, and sleep.
Hovewer, this is not so important.
Microlite20 is designed to be quick and easy to play. It gives to the GM a foundation for basics, and the GM is supposed to make quick ad-hoc adjudications for all the rest.
When adjudicating natural healing, answer these two questions:
- How potent natural healing is?
- Does it scale with level and/or stats?
Then you get all the possible options, how many hit points per day can you get due to natural healing:
- 1d4 (not so potent, does not scale)
- 1d6 (more potent, does not scale)
- 1d6 + STR bonus (even more potent, scales with STR)
- flat number (more stable, does not scale)
- 1/2-level, rounded up (not so potent but more stable, scales with level)
- equals to level (more potent, scales with level), etc.
As a GM, you can also decide that natural healing in your game is not the thing at all — in this case you say to your players, that one needs more than few days to completely heal a stab wound, or has to use magic. Just be consistent with your choices.