My Leyland Mini S came with an engine that has been replaced at some time.
It's number is 10YTAH4659 2
Any clues to it's origin.

yep most likely out of a mini

Morris 1100 is my bet.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Yes it is a Morris 1100 motor.
A Mini K would be 10YC/Ta/H
Mini Clubman/ Leyland Mini /Mini S would be 1000 to 1016

Thanks Morris 1100
I knew you would know.
Is there any difference in spec's between the original Leyland 'S' motor and the 1100?

Most of the differences would be in the gearbox, so hopefully they kept the Mini box. The Morris 1100 had 3 syncro and rubber unis, the Leyland Minis had 4 syncro and later ones had rod-change and pot joints.
The late 1100s fitted to later K, Clubman and Leyland Minis had head bolts instead of studs and nuts. This is a step backwards (and probably only done to increase local content)

Thanks for that.
I'll have do do some further investigation.
This is my first Mini, so i'm still finding my way.