Previous NOS 1256cc engines have turned up before, in the last couple of decades I have managed to obtain 3, or which all found a home soon after.
Two came together as short engines and I added the heads to make a near full engine and passed these on for not a lot less than the asking price.
The last NOS engine I got (10 years back) was a full engine assembled but no starter dizzy coil alternator (bare engine) is now sat in an owners pride and joy HC saloon and was passed on at 3/4 of the this asking price.
So to me £600 ono is not an unreasonable price considering to recon an old engine with all new bits and rebuild would cost me at least £400 these days, plus my time on top. Alas I have no need of one otherwise I would of shown more interest.
Out of interest does it look anything like the images below?
One of the two NOS short engines I obtained in 2007
My last NOS engine from 2008
I am sure with a little more information a sale would ensue