Hyundai Repair: 1999 TIBURON, head gasket, piston rings

Question
This vehicle has 300,000kms and compression has dropped.
With 300,000kmsCompression has dropped. Will replacing the Head gasket and the rings only [rather
than total costly retrofit] increase the compression
suficiently  

Answer
The head gasket is unlikely to restore any compression.  Replacing the rings may restore some compression, but that'll depend on the reason for the compression loss in the first place.

Can you tell me the compression readings?  One of the items of prime importance is whether the compression readings are similar to each other and whether the low cylinders are a little low, a lot low, or near zero.

In addition, if the reason for compression loss is that the rings are seized to the pistons, this is one of the few times that I'll recommend a chemical attack on the problem.  GM has a TSB recommending mixing their top engine cleaner with gasoline, putting the mixture in a pressurized contaner, and then running the vehicle on the mixture.  I've seen this used in the past with favorable results for the purpose of cleaning injectors and for freeing up carbon-seized piston rings.  While it's doubtful you'd be able to do this yourself, you may be able to find a local shop with access to the TSB so they can follow the procedure.