I don't personally have any experience with Microlite, but just looking at the rules, you pretty much don't need to change anything. Comparing the example monsters given in the Microlite document to the monsters on the v.3.5 SRD, the numbers are identical, even where they shouldn't be.
For example, let's look at the Bugbear:
Bugbear HD 3d8+3 (16 hp) AC 17 Morningstar +5 (1d8+2) or javelin +3 (1d6+2)
This is from Microlite, but is identical to the SRD. Why does a morningstar deal 1d8+2? Because it has 15 Strength. Why does it have 3d8+3 hp? Because it has 13 Constitution, for +1 hp per Hit Die. ...but there is no difference between Strength and Constitution in Microlite, so these should be the same number.
What this means, is that, if the creators don't worry about the difference, neither should you. Just use the combat stats for any 3.5 monster as written, and you'll be fine. The difficulty will be nearly identical (all characters in Microlite basically just have full Base Attack).
We do need ability scores for skills, though. For the three Microlite ability scores...
- Strength: Use whichever is higher between Strength and Constitution
- Dexterity: Same as the Dexterity on the SRD
- Mind: Take the average of Intelligence and Charisma.
As the rules state, they get a bonus equal to their hit dice to all four skills (+3 for our 3 HD bugbear), and then add an extra +3 to whichever skill you want that monster to use. You can even decide while playing, you don't even need to convert that. But in general...
- A monster with good Hide/Move Silently should use Subterfuge
- A monster that has lots of Knowledge skills or has a knowledge role should have Knowledge
- A monster that focuses on talking to players (e.g. a succubus) should have Communication
- All other monsters should have Physical
And that should be everything. Hope that helps!