This is he sort of thing that is curiously, seemingly, engrained in people. Why is converting the Scooby to FR and putting in a transverse engine like that Volvo lump more acceptable than putting a 908 engine in a "Honda Type R" (you mean like the NSX?
)?
This is a game, the general lack of spare 908 engines lying around and the supposed / imagined "difficulty" of sub-frame modification (never mind the fact that most "production-car-based" racing formulas had heavily modified or often bespoke subframes anyway) shouldn't matter, except maybe for cost of modification. Granted, uni-body / chassis modifications might need to be a bit more extreme in some cases, but that's what RMs are for, and I'd argue that the RM "system" could do with a bit more customisability / adjustability anyway. For the case of non-modified car bodies, a simple engine bay volume and dimensions with engine volume and dimensions intersection would define the possible engine / drivetrain swap options, whilst practically anything should go for RMs in the ideal case.
In short, it's still a totally arbitrary limitation. Is a 908 engine more or less hassle than, say, a Toyota GZ V12 installed into any given car? The problem is that there will be perfectly feasible engine swaps (in the grandest of grand schemes of engine swaps in general) that can come completely out of left-field and produce some truly interesting cars (as well as some downright ludicrous ones, granted, but that's the same argument levelled at the livery editor), and that shouldn't be obstructed because of what is essentially a matter of taste.