OK. Forget about the engine. You have a clutch pressure problem.
You need to acquire a 300 psi gauge and look at the main pump housing... and you will see a line coming off the bottom of the housing at an angle and going up to the transmission housing by the cab mount. There will also be another one down to the FWD transfer box if it has MFWD... Anyhow... find this line and look at the pump. You will see a raised gallery running up about half the height of the pump at an angle. There ~might~ be a 1/8" NPT plug in this gallery depending on the age of the tractor. Replacement pumps will have this plug as will most pumps on ~86 or later tractors. Install the gauge here... If it doesn't have the plug you need to remove the line from the bottom of the pump and install a 'tee'. Hook the gauge and the line back to the tee, then start the tractor.
You should find 160-180 psi here with the tractor warmed up, warm oil and running at a fast idle (850 rpm). You also want to run through all the clutch shifts which include DP high and low, PTO on and off and FWD engaged and disengaged. If you find one where the pressure is significantly less than the others it suggests a leak in that circuit (clutch pack). If you find that all are low or that all are slow to achieve pressure then you can expect a problem with a leak at the regulating valve or a weak regulating valve.... or slow fills could indicate an obstructed pump intake (strainer). This strainer is located towards the rear of the pump housing and you will recognize it by the 1/2" square plastic plug. Simply turn this out and wash the screen in varasol taking note of any chips or scum you find in it.
CHeck these things out and post back. Whatever you do... DON'T go and split the tractor looking for a clutch problem before you know where the problem is.