is there an inexpensive way to increase the gas mileage on my stock 1983 4.2 liter landcruiser engine and maybe more top end power?
First step would be to run a compression test to make sure the compression is where it ought to be. Adding parts to a weak engine will accomplish nothing.
After that, there are a series of small things you can do to aid the fuel mileage a bit although you will not get any astronomical gains.
Advance the ignition timing a degree or two, replace the spark plugs and air filter, overinflate the tires by a few pounds, and make sure the carburetor choke flap is not sticking along with making sure the carburetor does not “dribble over”.
The latter means looking down the carb. throat while the engine is idling and verifying that you do not see fuel being atomized from any discharge tubes. If this is happening it can often be heard as a faint crackling noise and can be caused by the float level being too high or an improperly adjusted carburetor.