Richard,

You are on the right track, but I would strongly suggest that to have an engineer look at the particulars of this case. The length of the span and the fact that the spire is also resting on the truss make this more than a "seat of the pants" sort of fix. The loads to be carried are large.

The steel plates and bolts are the right approach, but the loads need to be calculated to ensure there are a sufficient number of bolts. Shear rings between the plates and the timber could also reduce the number of bolts required.

I know there are products advertized in magazines such as Fine Homebuilding the claim to both stop rot and repair/replace the wood in place. I personally have no experience with them, but I'm sure the manufacturer would be willing to help.

Good Luck.