Running for 1 1/2 hours is most likely going to be something heat related.
The most common causes are the coil heating up, or as you mentioned, vapor lock.
Like Old said, you need to act quick and catch it in the act. Sometimes heat related problems are difficult to diagnose because by the time you get around to finding it, it has cooled down and corrected itself.
Start with the coil. When it fails, feel the coil. It should be about the same temperature as the rest of the metal in the area. If it is exceptionally hotter, possibly it has the wrong coil. If it's been converted to 12v, the coil would have to been changed or a resistor added.
Vapor lock is caused by gas boiling in the line. It is common on hot days under heavy load. The line should be steel, not copper. Copper is more conductive, can be all it needs to push it over the limit. The line needs to be as far from the exhaust manifold as practical, and as direct and down hill as possible. If it has an inline filter, not only can it be restrictive, but the fuel sitting in the filter can stay there longer, long enough to reach the boiling point. Best to let the sediment bowl and screen do the filtering and have no inline filters.
That tractor never had a heat shield between the engine and the tank, so be sure everything is right to keep the under hood temps down. Air flow is important, not just to keep the engine cool, but the externals also. Watch the radiator for grass, especially when mowing. Be sure the fan is straight and the belt is tight.
Retarded ignition timing causes high exhaust temps. Be sure the centrifugal advance is working, the points are set, the distributor shaft is tight, and the timing is properly set.