Question about GSX 750 F BJ 1993, does not start

Hello all, I have been the owner of a GSX 750 F Built for 2 days. 1993 The wear parts are all still in order. When buying, the seller (privately) bridged the battery and the maschione did not jump on. He swore to me several times that it was only drunk and I only have to dry the spark plugs and then it runs completely 100%, I bought the thing then, loaded it on the trailer and brought it home. At home the tank was dismantled, and the spark plugs taken out and made dry, let the starter run without candles a few times, re-screwed candles and …. it does absolutely nothing!! I georgeld and torgeld. After 2 minutes there is an infernal bang from the puff. So in the direction of cannon stroke. I have tilted fresh petrol, tried the bezinhahn on all positions. On “pri” runs the gasoline down through the inner life to the ground (?) is that normal? Meanwhile I have the candles again as usual dry laid t and it just doesn’t start. To the crowning conclusion I found that no more! Oil could be seen in the display glass. I then refilled about 500ml and now it’s on the center… Manomann. Can you tell me what to do/check next to make the machine start? According to TÜV report, the machine went over the Tüv a year ago without defects and since the only 30km was driven… Therefore I also believed to the buyer that it is only drunk. I ask for Hi lfe vg Claus