2000 Vauxhall Vectra noisey hydraulic lifters

Hello guys
2000 Vectra!
I have a vectra with a noisey lifter. My question is how do I tell which lifter it is? Do I have to remove the camshaft to replace a lifter? Could you suggest where to buy cheap parts for a vectra as i'm in australia and parts over here are very expensive.

And secondly I have a miss when at low idle. It's apparent that its an oil leak from the rocket cover gasket down to the spark plugs fowling the plugs causing the miss. I replaced the gasket twice but the oils still leaks. Could you make a suggestion on a better technique to fix the rocket cover gasket.

Also how do I adjust the idle, unlike the carby I am usless with fuel injection, but I think its a computer adjustment. Should there be another way I would love to hear.

Thanks for your time