[Closed] Fixing up corroded hope calipers

I've got a mates bike to fix up. He's got old (bout 12 years I think) hope mono m4 brakes on it. They are totally corroded with extremely stuck pistons, not sure if its worth trying to soak them in something (petrol maybe?) and see if I can get them cleaned up and working or just pick up some cheapish shimano brakes for him.

I know being hopes all the spares should be available but is it just gonna get expensive and too time consuming compared to picking up some slx brakes for 60 or 70 quid