The fix is a quick financial shifting of a portion of the closing costs. Since you do not have a lender involved, it should be a bit easier.
Chris is absolutely correct in that you need to get your attorney involved. In order to shift the costs, you need to have your attorney discuss the situation with the Seller's attorney so that you can see if they will agree with rolling that cost into the note. It will also require that the note and mortgage be rewritten and a new HUD prepared for your approval.
Considering you cannot afford the additional closing costs, I wouldn't recommend you get a real estate agent involved at this point, unless the Seller is willing to pay them for their time. It doesn't sound like you have the finances to cover the cost and the majority of the work that we perform is well before you get to the closing table.
I have to ask, since you didn't have a real estate agent, Did you hire any inspectors during the process to check various parts of the house systems?
Best of luck.