Dr.Jeff
True Classic
This is why it was suggested you might have a 1500 head on your engine. For that matter you might have a 1500 engine? Just because the car originally came with a 1300 does not mean it still is. These engines are very prone to failures and get parts (including major components) replaced all the time. Often with whatever they could get a hold of. And more so, swapping a 1300 to a 1500 is considered a easy performance upgrade.Mine definitely has the hole in the head.
Either way, if the head is a 1500 then the new valves you installed might not be the right size. And that hole in the head should be plugged.
Given you had milkshake oil and steam exhaust, and the uncertainty about what parts were installed, and the need to plug the hole, I'd want to remove the head again and verify and correct everything. It will be much less costly to do it now then wait for a failure.
To plug the hole all you need is a common thread tap and a common plumbing plug. There are several threads describing it in detail.