Its probably not a good idea to change multiple things at once.
The plug should be a CJ8 or an NGK BM6A. I personally preffer the latter.
If it is revving too high it is most likely sucking air in somewhere.
The Oring the carb goes on is the least likely to leak, its usually pretty oil soaked which would help seal. Its a common size, easy to replace.
The other oring where the inlet goes in the block is prone to leaking, of the top of my head these are an 015 size.
The starter O-ring is not an imperial size, imperial ones are either to fat or to thin. Its a metric size, cant remember if its 98 or 95mm. They are not cheap even in bulk from a bearing place so its worth just picking one up from a mower shop.
Is the Diaphragm in the carb seated propperly. This is a very common spot for an air leak. If its not seated/sealed right the motor will rev.
Removing the plate will effect the govenor.
The spinning flywheel crates suction at the vane on the block, that pulls on the diaphragm pulling the poppet back in, if the revs drop, less suction lets the poppet out. If you soften the diahragm and let it flex more allowing the poppet to move out.
