Why would there be smoke coming from tappet cover?

By not changing the oil you compromised everything. The oil is old and tired and now its got the gasket debris and else from the head swap. The tapping noise is a dud lifter or worn camshaft. You need to better describe where the smoke is coming from as it may just be spillage where you swapped the head getting burned off.

you have a blocked crank case vent/filter

if its white smoke and wet its a internal engine water leak

if it is black smoke it bis an internal engine oil leak

if it is blue smoke it is an internal fuel leak or ignition failure somewhere

Hope this helps have a great day

Has your car got a turbo ?
If so the seals my have gone in it.

EGR valve playing up ,just cap it off with nice blanking gasket made out of a coca cola can job done and sorted

If oil is on the spark plug tip then it could be from worn valve stems or from worn piston rings. When you accelerate away from the traffic lights and the engine blows a lot of greyish smoke then this is a sign of worn parts.
If the oil is leaking into the spark plug hole, then it could be leaking from the tappet cover, either the tappet cover bolts are loose or the tappet cover gasket is faulty.

You have a 4 cylinder and there are seals in the cover around the spark plug holes. You can replace these with a rocker arm cover gasket set. Costs about $25.00. The oil does not hurt the sparkplug, but over time it will affect the plug wires/ boots and cause a misfire. these are not hard to replace. Your call.

Sounds like white smoke is antifreeze. It might be leaking all of the time. Does your antifreeze level fall gradually over a week's time?

unsure what you are saying with car bogs down , if you have smoke coming out of exhaust pipes when you first start your car but o.k when running then your valve stem seals are need of replacing , if your car continue to blow smoke as you drive then your rings are gone and you will need piston and rings attended too.

if you only have oil on outside of spark plug , then your rocket/tappet cover may need a tighten or new gasket .

hope this is of some help ,as i'm not sure what you were descibing ...

tappet cover leaking oil-replace as necessary