Fe35d low power

If it is that low on power that it can't provide 15hp you'd notice it on the road (e.g. pulling a trailer uphill or even with tractor on its own.) If tractor seems ok I'd be questioning pump hp requirement or else its actual mechanical well- being. There's not a choice of gearing for the pump or anything odd like that?

If tractor is short of power and has just been rebuilt it could be many things. Make very sure air inlet system is clear (no old socks stuck in there. If its getting enough air, fuel system, valve timing or injection timing are possible causes too. Fuel lift pump has a filter screen that you could check. Check fuel tank cap vent hole is clear. You can undo inlet pipe to injection pump and turn the engine on the starter to see what comes out. Lots of diesel hopefully. Bleeding of fuel system will probably be needed afterwards. Were injectors also serviced? I would say to eliminate everything else before suspecting the injection pump. Its rarely faulty and is an expensive item to replace. (we did just that when the fault was a blocked air filter, which was very stupid and an annoyingly expensive blunder to make!)