[RedDevil]

That is a hefty engine swap then, doubling the original power.
Anyway, lookup the data on cars that can achieve 266 km/h, you will only find cars that have more than 250 hp. To make a road legal car that only needs 157 hp to do that is a fantastic achievement.
120 hp won't bring it beyond say 245 km/ h though. A very good result nonetheless.

What worries me is that this car was not designed to handle this speed, so it may be quite dangerous to try to do them without proper suspension firming and the like. When you test a cars ability outside its designed operating window, take small steps at a time and evaluate them thorougly.

The kammback design generates some lift. At normal speeds this plays no big role, but at very high speeds it may contribute to instability and bad controllability. It may need a spoiler... and some extra hp.

I am worried for your safety right now.