You're allowed to keep it and you're allowed to try and use it to buy stuff but you won't have much luck doing so.
Turning up with foreign cash would be effectively the same as trying to buy stuff with precious metals, jewellery, or rare Pokemon cards - while there's value in the item it's not immediately verifiable, it doesn't fit with the usual workflow of the shop, and they're under no obligation to even consider this as a payment option.
You'd have more luck using foreign cash for person to person sales where bartering and swaps are more of an accepted payment option. Even then expect dubiousness and to lose a lot of the nominal value when it comes to purchasing power.









I feel like you'd have be a philistine to do this to a functional proper classic. If the original drivetrain was beyond repair it'd be more acceptable though and an electric swap would be no worse than trying to shoehorn an LS in or something else similarly non-original.