Magic Items will give you headaches like you won't believe. AD&D has the most overpowered magic items of all games out there. RuneQuest has comprehensive rules for producing magic items, but they are extremely expensive, requiring a bit of the caster's own soul to power them. Needless to say, only an utter idiot would churn out magical weapons he'd never see again or rods to hold a bazillion spells he'd never cast.
There are two ways to deal with this and still be reasonable. When the Great Blue Wave comes over the world, and physics changes (game system conversion), you can simply disenchant all magical items (your players' and all NPCs'/villains') and make it a feature of the world. Everybody loses their magic items and has to start over, making them anew. Alternatively, you could permit magic item makers to substitute the POW of another for their own when enchanting, but at a lower efficiency. I kind of like a ten to one ratio, but the "donation" needn't be voluntary. Thus, a magic-item maker of a more-or-less goodly bent, could set himself up next to a butcher shop and buy the pig's souls from the butcher, much like his other customers buy ribs and chops. If a pig has an average POW of 5 or so, two pigs per one point of enchantment would keep magic items expensive enough.
You could even have some animals be more efficient than others. Maybe bulls and stallions count double, but only if they are very good quality. This could explain why a lot of gods prefer bulls and stallions as sacrifices.
If you take this route, you still need to modify the specific forms of the enchantments. RuneQuest does not have "always on" items. Instead, the items hold RuneQuest spells that the owner can then cast as if he knows them. Thus an AD&D sword +1 would be a matrix holding a Bladesharp 1 or an Intensity 1 Damage Boosting in RuneQuest. The user would still have to make an appropriate casting roll and expend his own MP to power the spell, but he at least doesn't have to learn th spell to use it.