I understand that, the last person with a ZS that didnt just want to buy an ST on here, had the conversion done and the car was ruined. So ended up buying an ST.

I understand the sentimental value but with such a risk as an engine swap that can go wrong, I would still buy an ST over risking ruining the car that means so much.
It will be hard to part with it of course but by doing this you can also remember your grandad as the person that helped you to move onto this next step, by helping you buy the next car on the ladder. In that way every car you buy can always help you remember your grandad as long as you remember where the original starting point was.