Q: Car wouldn't go over 50 mph I went over and next thing you know I hear loud knocking from the engine

The knocking is suggestive of a mechanical failure of some sort. I would first measure cylinder compression to get a basic idea of the condition of the engine. If it doesn’t test well (too low and/or too much variance) you needn’t do anything further as you’ll have to refurbish the engine. If compression is good, the cause of the knocking has to be pinpointed. If bearing related, there again, unfortunately will have to disassemble the engine and repair. The white smoke in the exhaust of course could be coolant. Sometimes just the head gasket can be replaced (if head and block are measured and confirmed flat) to resolve a leak but if you have a serious mechanical knock of course just putting a head gasket on is a pointless exercise as the whole engine will need refurbishment. To get the underlying issued pinpointed, and get a comprehensive explanation of your repair options and costs, please request an engine noise diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.