Welcome to Blaster Forum! I'm glad you found yourself a good deal on a used Blaster. They are so much fun to ride. Of course, it has to be made rideable first.
Even though this really isn't the place to post such a question, here's an answer for you. It's all good. I'm always learning every day myself. I:I
If the chain did indeed come off and punched a hole in the engine case, as the seller said, I don't have much in the way of good news. The damaged case half will have to be repaired or replaced, which entails tearing down the entire bottom end of the engine to gain access to the case half. Even if the damage can be temporarily repaired with some JB Weld, without removing anyting, there is a good chance that some metal shavings or pieces may have fallen into the engine. Any metal shavings left inside will promptly wreck a bearing.
However, you have options. A complete replacement bottom end that's ready to bolt in can be found for $ 100.00 or so on ebay. This is a very cost-effective fix, plus you'll have some spare parts to boot. You could also just buy a new LH case half, but this is a MAJOR repair for somebody new to ATV's. No offense, of course. Suggestion: Get yourself a Clymer or Hanes manual if possible. This is the best tool you'll have when you go to war or play with a Blaster.