FWIW I also have found heat the killer on the main bearing -- hence scoops to ensure it gets continuous air flow over it. For replacements we use NTN 6000 LLU for front and NTN 6904 LLU for main -- both are sealed and available from local bearing suppliers (down here in OZ anyway).

As for changing header pipe lengths -- I am really surprised. I have two in service for the past 4 years, with standard header and pipe, and never had a problem. These things are pretty damn robust. I have flown with stuffed bearings, pipes that have slipped within joiners (neither deliberately!) and even though it has had a minor impact on peak RPM -- nothing that makes them unfliable in the ways discussed here. And there there are at least a dozen other OS 140's in our Club alone with similar history.

While I may have been lucky, when I have had problems like the ones described here it has always been fuel side and my problem, either dirt somewhere it shouldn't be, or a blockage or a leak in the fuel system (one way valves, fuel tube nicked somehow during building/repair). 15% nitro, 20% oil, and an additional shim in the head (unless it is one of the newer ones) seems to be a bullet proof recipe.

This is is no way meant to suggest you may not have other problems -- just to suggest that changing header pipe lengths etc. as part of the standard approach to setting them up sounds a little extreme. They just aren't (usually) that fussy.