What's the biggest engine I can fit in a Mk1 Gti Covertable

Hello,

I currently own a Mk1 GTI Convertible. The engine is sweet at 115k miles but I'm considering making the car a sleeper.

I'm interested to know what is the biggest/fastest engine I can fit under the bonnet. Appreciate there will need to be modifications to make engine & box fit but I want to keep the exterior as standard looking as possible.

Any advice would be much appreciated.

TIA