Check your compression on each cylinder as you may be in need of a valve job. If you dont have a gage, removing each plug individually and holding your thumb over the hole while you have someone crank it will tell you if you are real low and which cylinder, if any, has bad valves. Also, as has been mentioned, fresh gas is important with some tactors. I have an old Allis Chalmers C that almost seems to double in hp when you get all the old gas burned off and some fresh in there. My 1951 Ford 8n, on the other hand, dont seem to mind what gas I use so I use it to burn off all my old or questionable stuff including that that has been mixed with 2-cycle oil for chainsaws and boat motors. Those engines also tend to be fussy with old gas. I have never had a gas tractor that came up short on power and was not caused by old gas or bad valves. I think you will probably be wasteing time and money with tune-up, plugs, or carburator work so I would look at compression (easy test) and new gas (even easier) first. Diesels on the other hand, get weak when the injectors get varnished over time, but can often be brought back to life simply by adding some diesel fuel supplement for a few tanks worth.