Here are a few natural Swedish rewrites of the title, ranging from most direct to slightly more formal:

Option 1 (Most direct & natural headline style):

Region Dalarna: Kirurgiskt instrument lämnades kvar i en mans buk under planerad operation

Option 2 (Formal/passive voice):

Kirurgiskt instrument kvarlämnades i en mans buk under planerad operation, uppger Region Dalarna

Option 3 (Everyday language):

Operationsinstrument lämnades kvar i en mans buk under planerad operation – Region Dalarna uppger

All three are correct, but Option 1 is the most common format for a Swedish news headline (using a colon to attribute the statement directly to the authority).