You changed a head gasket and 1 valve, then the noise started. Did you replace anything else at that time? The new valve would have increased pressure on #3, so could that have started the piston slap?
I'd probably replace the timing gear and keep this as a spare. If the new gear doesn't solve the noise, then you have a good spare gear. If the new gear solves the noise, then you can sell this mint gear on ebay.
