When those non-adjustables start a persistent ticking it usually indicates excessive wear in the lifter, the pushrod, the rocker, or all three. What has happened is that the preload is gone and the ticking noise is slack from the accumulated wear.
While you have the rockers off, inspect the pushrods. The ends should be perfectly round. Worn ones will look tapered or even pointy. If that's the case, look at the rocker cup. It too should look round and all of them should be uniformly sized. Pushrods and lifters are cheap and easy to get at the local parts counter. Rockers may take a little time to get by mail order.
Theoretically, you can change a lifter with the head on, but it's not easy to get those things out.