An inspector probably wouldn't care for it but it ought to be strong enough. That would triangulate the structure very nicely. Top plate to 2/3 of the way up the opposing rafter, joined in the middle. If there is no code and it has held up for some years, but you just want to reinforce it some I'd be inclined to do some kind of scissors rafter ties. If the powers that be want this to meet current code you will need to work with them.
Almost certainly would need a structural engineer to sign off on that one. The other would be to use the existing rafter as the top chord of some built in place truss. A doubled 2x8 might satisfy them or a 2x10. One would be to 'sister' the existing rafters. If the existing rafters are too small and you need to satisfy an inspector I can only see two solutions. Neither would substantially change the ability of the rafters to bear a 60 psf load. Ceiling joists that attach to the top plates & rafters & span the building do the best job of tying the walls together. Rafter ties, collar ties, whatever stiffen the roof structure a bit and help resist the outward forces that the roof exerts on the walls. I've never seen a roof spread at the peak except in a total failure, think hurricane or tornado. I hope PEG rides in to the rescue here 'cause I'm confused.
