Hello and welcome to the forum.
Just to clarify, is your car a 500, or a 500L. If the latter, this really belongs in another section, and I'll get it moved.
Some folks identify the 500 lounge as a 500l, so I'll wait to hear from you before actually moving anything.
Edit: it's now been moved.
is it possible to change the piston please
That depends on how much additional damage has been done to the engine. Almost anything is possible when rebuilding a broken engine, but if it needs machining, welding or sleeving, it's specialist work, beyond the resources of most garages.
garage says new engine but can’t find one anywhere thanks in advance
A new (or factory rebuilt) engine can be had from Fiat but the cost would be prohibitive on a 10yr old car.
Secondhand engines can be found by searching the internet; the problem is knowing whether you're buying something decent, or a piece of junk.
Most breakers will give you some sort of guarantee on a used engine, but it'd be highly unusual if that guarantee would cover any labour costs associated with fitting it, so it's always going to be something of a risk if you go this route.
Assuming you're not going to do the actual job yourself, do you have a good relationship with a trusted garage? Once it's been dismantled, if the replacement engine has issues, or if any further faults are found, you're essentially at the mercy of whoever is doing the work.
If you don't have somewhere/someone you can trust, you might be better advised to sell the car as it is and put whatever you get for it toward something else.