Sorry to hear your overheating problem. As to your choices, I would first find out the damage to the engine before thinking about replacing, swaping etc. I hope that you or a local mechanic can determine if your engine can be fixed in place, head gasket replacement, cooling system parts can be done with the engine in place. If not repairable in place, then a running/rebuilt A4 might be the easiest fix. Swapping to the a diesel would probably more expensive as other related things MAY have to be changed, fuel filters, engine beds, maybe even the fuel tank. I made the A4 to diesel swap about 19 years ago, love it, better hours per gallon etc. BUT, I miss the smoother, quieter power of the A4 (compared to my current diesel). Best advice I have is take a few deep breaths, get the total damaged assessed and start listing your options and costs to determine your best path. Good Luck!