I'm american, and I don't want to see every car with a small block chevy (SBC). I think the motor is the soul of the car. I don't care if your buddy put this in that. When you tube frame a car, it isn't the car anymore. You can drop a tube frame and a motor in any car and make it compete. There is no way that should be supported.
And what if it indeed changes the soul of the car ( in the same way as bolting a large Turbo charger onto a N/A car's engine will change or possibly detroy the soul or natural characteristic of that car, which also already is a possibility ) when the option always remains to keep it close to original or have at least the original engine still in place?
If it indeed becomes an entirely different machine with its own character not resembling the original car or even resembling anything close to what it was originally intended to be, it simply becomes another car in the game.
The only criteria if it should be offered in a simulation game is if it's a realistic proposition and happens in real life which it does.
The original car ( with 'soul' intact ) still is available to both the ones who'd also like a different version with a different engine as to those who won't use an engine swap feature in the first place.
In fact, the argument you use that it becomes a completely different car is not only accurate but the whole point of an engine swap feature in the first place, which in effect increases the car count in the game significantly ( only ofcourse mechanically and physics wise ).
Yes a lot of cars are largely, or entirely, defined by their engine but also a lot of cars are not ( even some iconic cars never got the engine they deserved ).
There ofcourse need to be limitations, both technically ( as I also don't like the prospect of the infamous AWD Vipers ) and a certain cultural sensitivity when relating to those cars where it would be simply sacrilege to alter them or use them for donor parts perhaps.
There always remains the option to go for purity ( even to those who will use this feature ) or keep the original engine.
And I think the drivetrain possibilities need to be limited ( instead of making some cars AWD, I think it would be best to keep all cars either FF, FR, AWD, etc. like they are originally, maybe only the ability to change some AWD cars into FR but not the other way round ) and maybe only engines from the same parent company and only when it's technically possible to do so in real life.
It will only be an addition, not take something away ( as the core of the game for you won't change, with probably special online events for engine-swapped cars only and vice versa ), and what if I would destroy the soul of the car? It's still just a game after all and my decision to make.
I could argue in some cases I've indeed replaced or improved the very thing you might consider the soul as for me it's just a mechanical upgrade with its own unique soul.
It's not the same as being Dr Frankenstein, and even if it were, what's the big deal?