How much power do engine mods yield?

First, try the free things like advancing the spark as much as you can without pinging up hills, and adjusting the carburetor idle mixture screw. Also, insure the choke pull-off works or you will stumble around when engine is cold.

If you ever take the exhaust off, weld an O2 bung in and plug it until you get an O2 monitor. Pulling spark plugs to judge O/F is tedious and inexact.