I was looking through the GCR trying to figure out what is legal and what is not in ST, and think the rules may need work.
Alternate engines of the same manufacturer (Honda/Acura, Nissan/Infinity, Toyota/Lexus, etc) are legal, but next to nothing is written on it.


G. Engine
1. Alternate engines may be used, given that if the manufacturer of the vehicle and engine are the same (e.g., an Acura
engine installed into a Honda car) and was available in a car delivered in North America. The chosen engine must
retain its original cylinder head and intake manifold. If an engine from a front wheel drive vehicle is installed in a rear
wheel drive vehicle, alternate OEM intake manifolds may be considered.


I can't believe that is all that is written on the use of alternate engines. I was expecting pages. I have done tons of swaps in my day, SR's, B, D and K's in Hondas, 5 lug swaps, brakes swaps and they are never just cut and dry.
You can't just make a blanket statement "alternate engines may be used".
If alternate engines can be used, updating/backdating really has to be legal. Putting an engine from a different model of the same manufacturer most times requires updating/backdating.
Driveshafts/axles may need updating/backdating
Subframes may need updating/backdating
When subframes are updated/backdated it usually now means the suspension arms will also need to be.
The steering rack also may need to be updated/backdated.



Looking at the brake section:
E. Brakes
1. OEM brake systems must be used. Alternate OEM brakes rotors or calipers from the same manufacturer will be
considered.
If OEM brakes from the same manufacturer is allowed, which I really think it should be. OEM brake upgrades are cheap and effective.
Spindles, forks, hubs, and arms may need to be updated/backdated.
Proportional valves, brake boosters, master cylinders, abs components, and e-brake components may need to be updated/backdated to safely run the upgraded brakes.
Updating/backdating these parts really doesn't increase the cars performance at all, it just makes it possible to run the alternate engine and/or brakes correctly as the manufacturer designed it. If you are allowing the performance upgrade (engine/brakes) you really need to allow the parts to have it done to factory spec.
As long as the parts* were available on a 1985 or newer USDM car, from the same manufacturer, unmodified to make fit (should bolt in factory fit), then I think they should be legal.
*subframes, steering rack, driveshaft/axels, arms, spindles, forks, hubs, proportional valve, brake booster, master cylinder, abs components, and e-brake components.