I think it's a much better idea. buy a Cupra and dress it down so it looks like a pile of crap ordinary Leon. If you remove the Cupra bumpers, wheels ect you can
store them and they'll be kept good ready to go back on when you sell the car. The flip side is that the ratty bumpers on the car can get scratched and damaged and you don't need to worry about it! In fact, if you put damaged bumpers on the car it would
help the ratty look and they'll be dirt cheap to buy.
That would be a lot of fun. A Cupra that looked like a junk bog standard car.
Your current plan just makes no sense to me. You're now talking about fitting fancy bumpers etc, this is not a sleeper, and I really don't think you've thought the performance aspect though. I'll bring up a couple of specifics.
BMW used essentially the same pistons in multiple engines. 4, 6, 8 and even 12 cylinder engines and the pistons are interchangeable. However, although they fit, there were three different
qualities of piston. The higher powered cars got the best quality pistons and the lower cars got ones which were cheaper made as there was no point in spending money on parts that are better than they need to be.
All manufacturers work this way. Even if the block and basic engine design is good for big power, like the Nissan SR20 or the Cosworth Ford put in the Sierra, you still need to upgrade a lot of other parts. The 1.4 TSi is built for 150 bhp and not just the the engine and ancillaries but the gearbox, transmission etc too. None of these parts are designed to take double that power. And they won't take it. For a while maybe but not for long.
Another aspect is cooling. More power equals burning more fuel equals generating more heat. The larger engine in the Cupra isn't just so it can burn more fuel but so that it can deal with the heat better. Bigger cooling channels, thicker metal and the whole rest of the cooling system is more powerful to deal with the extra heat the thing is going to generate. Bigger vents and ducts in the bumper, you could go on all day.
Basically, if you want a car that worries a Cupra, you're going to have to build a Cupra! If you simply up the power on a 1.4...it's going to break. Maybe not straight away but eventually, bits are going to start failing. The car will have cost a fortune to build but it'll spend more time on the drive than on the road til eventually something big inside the engine lets go and it's game over. One very expensive pile of scrap is what you'll own.
I'm not trying to be a downer here, I'm trying to stop you doing something stupid. Sure, loads of people will tell you to go for it but they're idiots as well. Throwing money at crap cars trying to turn them into super-cars is a game for the not very bright! Typically ends one of two ways. Car parked on the drive with failures too expensive to fix or, if it still runs, it runs like a dog and you can't give it away. I've seen it many times. Knew one guy who put thousands into his car until he basically ran out of money and it still wasn't right. Ended up selling it for a fraction of what he'd put into it and the next owner put thousands more into it! I had a guy mailing me every day begging me to buy his modded Focus ST as he could not sell it.
You
can do what you're planning if you have no limit to your funds and capabilities. I know two people like that, it can be done, but if it's a daily driver and your money has a limit, honestly, don't even start it.