d20 Conversions

For monsters, d20 and B/X conversations are relatively easy and can be done as you play. First, you need a 3.5 or Pathfinder 1e monster manual for the stats, and you can use the ones online, such as Nethys.

The GURPS Repository also has a few good notes on this procedure, which is one way to do it.

I start by setting the ability scores, which transfer directly over from 3.5 or Pathfinder 1e. I use the creature's rolled hit points for hits. Some sources say to use STR as hits, so make a note of that fact and adjust accordingly. 

Make some of the weaker "group" monsters (goblins, kobolds, swarms, etc.) like the D&D 4E "minions" - they take any damage and get taken out of the fight. Reduce the size of encounters and replace a few minion creatures with one specialist.

For armor, use the AC and find a similar set of armor in the GURPS rules, and set the DR (0 to 8 typically).

Attacks? Find the nearest melee weapon and use that. Be careful with multiple attacks unless the creature really needs it.

Also, remember high parries slow down combat, so be careful setting those defensive values too high!

Special attacks with saves are ability score rolls, and the rules in the GURPS repository for modifiers based on DC work well. A lot of the other special abilities work as-is. Some special attacks can be replicated with standard weapons, such as a flamethrower for dragon breath (full or half strength, depending on the dragon's size).

Skills you will need to guess, 10 + HD for primary attack rolls is good, but adjust that for balance.

You do not need to be perfect! Monsters are all different. A lazy cave dragon who sleeps on piles of gold all day will not have excellent combat skills, but a war dragon will.