Damage from diagnosis time while hunting for Ticking:
1. Let's suppose it is a bearing - if the load is light (idling or no more than 2000 rpm) not much more material will get worn off .But the engine will need rebuilding anyway so it is a moot point.In theory you disconnect the coil/remove(?) plug on the suspect cylinder to hear if the ticking is reduced(rod bearing) -but ask about this first! This may not help if it is a main bearing.
2. Let's suppose it is a stuck lifter - they usually are collapsed and jammed so there is a large clearance between the camshaft lobe and the top of the valve stem. Every cam rotation smacks the top of the valve way harder than any of those components were designed for. Not good. So move quickly with that Engine Ear !
If it is lifters ,yes you can replace them with the engine in place but it is a horrid job for the usual reasons- access. and that bead of sealant on the cam covers....
You'll need special tools(or make them) to hold the cams when you remove the covers.
You'll need to decide on replacing just the collapsed lifter(s) or all 12 on the noisy side.And they need to be heated in a pan of oil before fitting.
Hopefully it is something simpler.