What’s a complete TV175 s2 engine worth?

I’ve been looking for bits for my TV175 S2 engine and have come across a dealer with an entire engine for sale. I am going to go have a look at it and he is due to send more pictures of it tomorrow. How do you put a value on something like this if it is original? I completely understand all the caveats and will need to make sure it is original. I will post some pictures when I’ve got them...
Thanks
Thanks