Running an AD&D adventure ("The Ghost of Harrow Hill") in 5e; any advice?

[Unoriginal]

The final boss is a monster that didn't make it past 3e, so I'm just using a Deathlock, but I'm not sure if I'm missing something. The villain of the adventure's the ghost of a wizard, and the final battle is him forced into his decaying former body. The original adventure basically makes him a melee only undead, and I'm wondering if that was the point, and I'm losing something by using a spellcaster undead.

Is the wizard able to cast spells while in ghostly form, but the final boss is him being cut from his magic by being forced into his corpse?

I'm a little worried about treasure too, since AD&D hands out +2 and +3 weapons like candy. I was thinking of just nerfing the +2 and higher weapons down to +1, or replacing them with uncommon magic weapons, but I'm not sure that's the best approach?

That would likely be for the best. Common and uncommon magic weapons can be pretty great and flavorful, and a +2 or +3 weapon in 5e are very significant boosts.

You can also remove some of the weapons, on top of that.