Its cheap enough just to buy a new pump.
I had a OG VW pump on mine that leaked like a seive on the way back from cornwall last year and after looking around to find the correct gasket set i just fitted a sealed unit pump. 10 mins and your up and away again.
But as said change the oil 1st, then also change it again in about a 100 miles or so just to be sure. I would rather run it and change the oil than be tempted to use a flushing agent