Right my "mate" went through this years ago BUT it WAS a ringer.

He bought it very cheap (mk2 gti) and had all the logbook etc but after changing the wiper motor spotted the panel had been cut and welded.

He ground the numbers out to blank the shell,pulled the VIN off,numberplates etc and broke the steering lock,drove the car to a quiet set of garages,and reported it stolen.

He then rang the police a day later saying he'd found it locally with little damage but it looked like the id had been tampered with.

Police came out and he told them he was restoring the car hence had no tax Mot or insurance on it.

They wrote a letter that they had no interest in the car as being stolen and he kept his original plates with no issues whatsoever.

That car went round for years with no issues and may still even be on the road.

Me and a mate went to look at and he bought an xr3i registered "series 1 turbo".

The car turned out to be a mk4 rs turbo with a mk3 front end rung to an A reg 3i.

I pulled the carpet back and found the numbers not even welded in straight and we were set to walk away when he also produced a letter from the police stating they had no interest in this car although to me it was a clear ringer!!!!

My mate bought it for £500 with tax/test and drove it for 2 years,went through 2 mots and was sold on again with no issues.

I'm trying to say obviously the police are concerned about rung cars but an old fiesta someones gone to the bother of swapping an engine and all glass into is probably so far off their radar they won't even bother to send someone out to look at it.

Put a blank floorpan repair piece in and be done with it,if the shells clean that's all anyone cares about,you've got the v5 as well,matching numbers,glass even so just go for it.

One other thing you could do for £30 is hpi it,that revealed why an old Rs turbo I had years ago had a SABTVR number stamped on it as it was stolen recovered twice.
Never bothered me or who I sold it onto.