Do the simple stuff (oil,filters,linkages are correctly adjusted) first. You can partially conpensate for a worn multipower with correctly adjusted multipower shift linkages.
If you need to break the tractor, then it gets expensive. There are 2 things that that cause no high multipower. First, the multipower clutch pack is worn out, Or secondly the "nose cone" may be worn out. It mimics a worn clutch pack and you can't tell until you go in and pull every thing out. The multipower clutch pack is at the back of the gearbox and the mechanic said it comes out the front. The gear shafts have to come out first from memory.I had mine done by the dealer about '98/'99 and the real trouble was the worn out nose cone which was remanufactured. The original multipower clutch pack went back in as it was in good condition in spite of 3 or 4000 hours since fitting in the mid to late 70's.
Multipower gearboxes like oil. A trick to extend the multipower life is to take the tranmission dipstick and measure the distance from high to low levels marked, then measure the same distance above the high level indicator mark and add a new "high" mark. This was done by the MF dealer mechabic who did the multipower replacement back in the 70's