Congratulations, was that on Gumtree ? I saw one on there and wished I lived closer. Nice piece of history you have bought, it does look to be complete. Freeing up the motor depends on how bad it is, I have freed a few but they have been left to sit and surface rust in the top of the bore was the problem. You will have to pull the motor off to have a good go at things, I think the the head and bore would be one piece. You could start with some penetrating oil of some kind down the plug hole and after letting that sit for a while give it a try. If no luck try some gentle heat then cool then more heat and cool.
Hopefully it will free up and the bottom end is ok. Below is an interesting thread that may help in your restoration
https://www.outdoorking.com.au/forum/ubbthreads.php?ubb=showflat&Number=26015&page=1Hopefully members can add some of there successes regarding freeing up a seized engine.
