To replace an engine in a PA28 is likely to cost north of £15k, to replace an O200 in a permit machine can be done for a few thousand if you know the right people.
Unfortunately, the difference between an ownership full of aggro and an ownership which is fun is very much down to knowing the "right people" and this is as true for CofA as for a lawn mower
It should not be like that but GA is full of people ranging from well meaning but incompetent to outright crooks and it takes a while to put together a bunch of people one can trust....
A plane is not a BMW which you can just drop off at any BMW dealer...