Valve gone or piston or gasket? Which? '93 Escort

[balaclava]

I have a '93 Escort. Accellerating out of a service station on the motorway I went too fast in first gear, over-revved. Something popped!
I think I have blown either a gasket, a valve or a piston. One mechanic says a valve another says probably a piston with scoring of the cylinders? He says it would be better to go to the junkyard and put in a new engine. He will do it for £150 plus £100 for engine. I drove a further 200 miles at top speed back down the motorway. At speed it was doing ok. Surely if a piston or valve is gone it wouldn't do 90 mile an hour.Sounds like it is running on three cylinders when it is idling. Doesn't idle well. Stops if I take my foot off the gas. Changed a cracked spark plug. Some smoke coming out of spark plug hole. I know little about engines as you no doubt gathered!
Any suggestions?

[GTP Dad]

Pull the plug where you see the smoke coming out. Look at the plug to see if the plug is damaged. It is possible that the plug was not tight and damaged the threads causing you to lose compression. Do a compression check on the car. If it is a valve or piston the compression will be low on the cylinder that has a bad valve or damaged piston. If that is the case the best way would be to pull the head to see what the real problem is. A new head is a lot cheaper than a new or used engine. If it is just the threads they can be repaired and the plug replaced restoring the engine to normal.