I do not believe I've posted on this forum before but it appears to be the most expansive

I know it is probably in need of new valves as the timing belt was way off in the car. The PO also said it was burning oil when it ran last so I just have a few questions I am hoping to get answered.
Firstly, the oil leak is to my best knowledge going to be coming from the piston rings, but I am not sure. I have never rebuilt an engine before; from what I read the FIAT twin cam is pretty simple. Can anyone confirm or provide some insight onto the skill level of an engine rebuild? Any advice for a rebuild at all would be great!
Secondly, if I were to do an engine rebuild, I think I would want to increase my CR. I have read about fitting older engine heads onto the 1995 cc engine but can anyone attest to this? I am also thinking about higher CR pistons but that seems like a whole other can of worms.
Thanks guys!
Michael