Can deactivated weapons be converted to airsoft? eg. K98 or mg34
Not in the UK they can`t 
Aye. Deactivated Weapons cannot be made to fire anything through them again.
Its true to say in principle a deactivated stock is ok to use as thats never had a projectile pass through it although I suspect ripping it off a deact is a little iffy (not to mention expensive). So at least best to steer clear of barrels & receivers.
No longer involved in airsoft.
Thought about it myself, i went and had a chat with the local police firearms officer, and he said' basically.....if you take the barrel out or deep hole drill it or tamper with in any way, it immediately becomes a section 5 firearm, yep i know what ya thinking.....ALL I WANT TO DO IS MAKE IT INTO AN AIRSOFT GUN...... 
sorry nothing you can do I'm afraid, as soon as you tamper with the barrel, it becomes a working weapon.
This is for the UK.
