The GT1 "shareable parts" feature was not really related to the fact that hybridding was more difficult in GT1.
In the GT1 garage design there was a single map with elements to indicate whether you had purchased parts for a particular model of car. Now, each individual "garage entry" did have a separate indication of which parts you had fitted. When you went to change parts, the game would consult the appropriate entry for that model to see if you had the parts available.
Permanent mods, ie. weight-reduction, race-modification, and engine polish and balancing, needed to be recorded in the garage entry only.
The key difference with GT1 hybrids, as far as I can tell, is that the parts themselves had characteristics there were affected by the car they were being applied to. I think this is especially true in the case of the "base parts" (i.e. ones you don't buy, such as engines). Whereas in GT2 the effect of putting, say, a TVR Cerbera engine into a Mini, was predictable because the engine retained the same characteristics no matter what car the game found it in, in GT1 the "engine" part and its characteristics was less easily identifiable, and the characterics wouldn't necessarily be the same in another car. Essentially, it would appear, the developers for their own purposes in GT2 simplified the building-up of a car, to make modifications easy during development. A side-effect of that was that hybridding was a lot more predictable. A lot of good hybridding is done in GT2 by interchanging only the parts you usually can't change at all; base engines, drivetrain type, etc.
The "shareable parts" feature of GT1 does have a few strange glitches. It is nice that once you buy a part, it's there for all instances of that model of car, even if at some point you sell all those cars and buy some back later. But, when you buy a car via memory-card trade, a couple of things are wrong. First, you need to pay for all the fitted parts even though you might already have them available. But, even worse, the parts for cars purchased that way are not properly registered, and so if you had not previously purchased them, if you remove them, you can't fit them unless you purchase them again! So ideally you remove all removeable parts from a car before you buy it via memory-card trade, but that is difficult-to-do.