A guy on some other forum I frequent had an interesting post regarding the new 'Monster Vault' supplement for 4E.
Conversion is relatively simple. Subtract 1/2 Level + 4 from AC. Aim for an AC between 11 and 18. It's okay for some elites/solos to peek into the 20-22 range. Hit Dice = Level. Thereby determining saves, attack bonuses and hit points. This means a Level 13 monster in 4e probably only has around 52 hit points in LotFP and only a +13 to hit (usually good enough). And saves as a Level 13 Fighter. Damage is more complicated. Divide the number of damage dice in half. If there's any remainder add it back in and lower the die type. Also divide damage bonus by two. So... 3D8+7 in 4e becomes 2d6+3 2d6+4 in 4e becomes 1d6+2 1d8+3 becomes 1d6+1 1d4 +1 becomes 1d3 + 0
The damage adjustment appears to put the 4e monsters roughly on par (damage-wise) with the monsters in BECMI. Pretty close, at least. With a few outliers.
Powers are tricky and require a bit of finesse in determining whether the monster rolls to hit or the target rolls to save or both. I haven't nailed this down to a science yet, except to say that attacks to AC are attacks to AC and attacks to other defenses are likely candidates for Saving Throws. But I don't mind flying by the seat of my pants as long as I can keep it consistent for the players.