I've blown up my disco (melted piston)

Had a friend sort my blown head gasket out 3 weeks ago, he didn't tighten the head bolts correctly and it's gone completely.

I bought new water temperature sensors and these failed me apparently my temperature was perfect whilst the engine was melting itself.

Anyway I removed the head and yep no.4 piston has melted. Gutted.

I've managed to unseize the old girl but my question is am I better off replacing the pistons and repairing the head or throw a new engine in her?

Thanks in advance.

It's a tricky one. If you repair it, you don't know what other damage has been done. Having said that, what a learning curve. You could get a new engine, but again, you don't know the history of it. You pays your money, you takes your choice.

usual cause of piston melting is faulty injector, unless you mean its just picked up on bores