i had something simmilar happen to my engine but no marks like that. mine melted No 4 piston, bent no4 exhaust valve, cracked the head across every port and into plug holes, and No3 piston fell to pieces when i took it out.
i would advise taking all pistons out and check them inbetween the top two rings cos a big chunk fell out of mine with no other damage to the piston.
if the engine is nearly std i have a brand new py piston and good SH injectors that i tested at work.
I blame my injectors cos after replacing them never had a problem.
If your pistons are 1341cc you might get a single one from Agra precision engineering in dundee 01382 229333. cost approx £200