I've got a mako king 165 gathering dust

Reckon it's possible to attach a foil to this board....?

yes

Are the makos are a high density foam core, divinicell or similar? If so you'll need some oversize resin/filler plugs around the bolt holes to prevent compression from the bolts and probably a couple of layers of glass on the top and bottom, with a few mm of filler on the bottom to strengthen the load bearing point where the front of the plate sits and to flatten out the concave. In other words, it will be forever compromised for its current use, but i reckon it should make a good first foilboard.

If wood core, less reinforcing required, just do something to keep the water away from the wood and make a flat section where the plate sits.

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theDoctor said..
I've got a mako king 165 gathering dust

Reckon it's possible to attach a foil to this board....?

Yep. Just couple of rectal exams and you will have recovered the cost,