What engine have I got?

Hi guys I'm new to the forum was hoping some of you could help me. I've recently acquired a chopper with some issues. I'm trying to establish what engine I've got. I've put the engine no. Online to establish what model no. I've got and it tells me it's a 75 z1 900 cc. However on measuring the bore size which is nearly 70mm this contradicts the 900 cc spec which should be 66 mm. The cylinder head exhaust studs are 8mm and I believe the 900 are 6mm. Sorry if I sound stupid just want to clarify what I have got. Also if anyone is from UK can u recommend a specialist cylinder head repair for spark plug hole Pics attached

Engine may be a mix of 900 and 1000 parts (pretty much all the same stuff and mostly interchangeable) and the engine number only confirms the age of the cases.
The 900 liners can be taken out to 70mm on rebores so it could still be a 900 ( look for the capacity stamp on the front right of the barrel to confirm either 903cc or 1015cc.
900 and 1000 heads are the same casting , same size valves etc with the 1000 having 8mm studs but many 900 6mm studs got took out to 8mm if they got stripped.
Any good engineering shop can fix the plug hole or just buy a kit and do it yourself.
For better help in the UK you should join the Z1OWNERSGB forum,.
We welcome any big Z , stock or modified, chops, streetfighters etc.