I’m sure there’s a rediculously obvious answer?

I'd be more worried about those push rod tubes.

Are you Joe Kenright?.

Ok, fair enough. Right size spark plug tool must be available somewhere.

Even if you get one with the right size for the plug but too fat to fit in the outside, then grind it down so it works.

Looks to be a 10mm plug, a jap bike plug socket should work.

It’s supposed to be an evolution engine in an 88 tour glide classic

There is this type that may tilt in behind the pushrods.

https://www.ebay.com.au/i/121990179245?chn=ps&norover=1&mkevt=1&mkrid=705-139619-5960-0&mkcid=2&itemid=121990179245&targetid=920064925813&device=c&mktype=pla&googleloc=9070869&poi=&campaignid=10101785177&mkgroupid=102311925580&rlsatarget=pla-920064925813&abcId=9300368&merchantid=264413199&gclid=EAIaIQobChMI0ba5q4CN7AIVw34rCh2LxAm-EAQYBiABEgJYIfD_BwE

I have seen plenty of heads that have been machined with no provision for service. That looks like a S.T.D or similar evo head, pretty much all of the aftermarket heads that were cast and machined by the manufacturer were accessible.

I had a set here recently that were machined in a way that only made sense to the genius that did them. Heads can be machined for 10/12 and 14mm plugs that have a number of different hex sizes, get in there with a caliper to determine its size.