I wouldn't recommend the weld it up option as these subframes rot from the inside out.
If its gone so bad that its failed the mot it will already be in a weak state and no amount of patching it up will last long or give it strength.
Bear in mind that your rear wheels are held in place partially by this subframe..so strength is a necessity..
Take procomms advice and renew it..especially if its a rare c2.
A good search around breakers for mk2 mondeo's can give great results...i found a new one (still with stickers)which had been put on a poor old decaying mondeo...cost me £30

The trick with breakers yards is to look for yourself. not ask them to find one.
I managed to cut mine free of the dying mondeo in just under an hour.
If you need help with any of this ask....were all eager to save a c2 here.
Good luck friend
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Wow ,a complete frame and from an st200 too...
im sure this will have the larger roll bar too..
Good find!