I like Jordan's suggestion a lot. The split tapered collet will be squashed down onto the bolt and there would be no scope for relative movement. I think that this is the ideal solution IF there is enough clearance for the slightly taller head that will result.
As for using standard countersunk Allen screws - I think that the standard countersunk, flat top head will be rather larger diameter than will fit into the recesses in the timing gear. If this is so it may be possible to turn down that extra diameter and leave enough parallel portion to drill for a lock wire. Doing this will not compromise the strength of the screw as the modification is just to the outer end of the head. I think that locktite would be a very bad idea as removing them would be near impossible if they did fail.
Another option would be to turn a small taper onto standard cap screws. I think that this could be done without getting too near the bottom of the hexagonal hole (compare with a countersunk screw ?) but the bearing area to the gear might not be big enough.
Of course the other option would be custom made CNC machined bolts to the original design but in a more modern high tensile steel. They don't need airworthiness certification and they are quite simple so it is do-able.
I don't think that these failures are age related but simply that the original design was too fragile. It may be that the bolts that are failing now were not torqued correctly but we have no way to check this. I know of several engines that have failed this way and been repaired "with standard Allen screws" but I don't know how they overcame the taper issue and am not now in touch with the people that did the repairs. I have not heard of one of these repaired engines failing again but that proves nothing.
David
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