Well the Kingdom by then had basically turned into the land of oppourtunity for ne'ardowells from Europe, like Guy himself, to get a bit of the wealth and prestige but Saladin wasn't unstoppable and at times not even that popular with the Saracens.
I think totally changing someone's personality is a bit of a stretch, as it have to happen pretty early on in their life and by then they might not even want to go down OTL's road.
That said save King Baldwin IV from his leperousy and you have the man who can save Jerusalem. After all he smashed Saladin's first invasion of the Kingdom when he was only 16 and was a very gifted ruler. Then again this might go against my above ideas, plus arguably his disease probably had a massive effect on his personality and hence his statecraft, maybe a healthy Baldwin would have been as bad as Guy?
Not to push my own product but King Henry II was offered the throne once Baldwin died, as were his sons but by then those that remained intended to cling on to their land in Europe. My TL has Richard Lionheart in a more accepting mood

. Anyway, Henry II offered to lead a crusade as a consolation but died before carrying it out. Richard picked up where he left off in 1189, but if there hadn't been so much family intrigue and delay the crusade might have set off several years earlier, and even if none of the Angevins took the crown, they might have saved Jerusalem from Saladin.