[dmjlambert]

The carburetor should have been put on with dry gaskets, no RTV or other sealant. If that was done then you don't need a fresh gasket if the old one is in good condition.

I recommend change the oil and put the new carburetor on there, then try starting it.

I recommend don't take top off of the carburetor while it is installed on the engine. Remove the entire carburetor, keep track of bolts and cotter pins, etc. and immediately cover up the intake hole so nothing falls down in there. Then take the old carburetor to a workbench and work on it later.