havent had much to do with OHC stuff before & have a suspected blown head gasket & rattly tappets. Anything especially problematic when sorting it & what configuration are the timing marks?
BTW its worth doing as its a ute conversion done very well about 15 years ago

You converted a Telstar to a ute?
Im interested to see that.

Most the time the tappets get noises as the lifters havent been cleaned well.
Generally a good oil flush and new oil solves most problems.

But if they are noisey due to high km's you can pull them out and clean them and place them back in or replace them.

Most people give the oil flush a go.
If the car sits for extended periods with no oil in the head they can become noisey.