Is there a way a DIYer can replace Mini valve guides or is it a machine shop job.

Please give me some clues.

You can drive or press them out from the top with a stepped punch, I made one from an Unbrako shoulder bolt.

Fit new ones the same way. Use a bush to set insertion depth.
But you then should ream them with a 9/32" solid reamer as they can close up.
The seats will need recutting too. Because many guides are not machined concentric.

I replace my own guides then get Graham Russell to cut the seats.

If you don't have the gear take the head to someone who does.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Is this the type doc?

Yes that is them. Not sure of size or machined dimensions I will measure tomorrow.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Sorry, been busy fishing etc... time got away.
Here is a pic, I can't draw with CAD anymore.
Please excuse my mixed measurements, as I only have imperial micrometers.

The shoulder bolt shank was originally 1/2" or M12, I don't remember which.
The biggest diameter is machined down to 0.446" x 42mm long.
The next diameter is 0.280" x 34.5mm long.
IMPORTANT- undercut the corner radius or you will damage the guide during insertion.
The short diameter at the end is 0.273" x 15 long.

The bush is brass, but steel would be better. It is used to limit the insertion depth.
OD ~19mm, ID ~14mm, length 56mm. Diameters are not critical, but the length is.

Guides are pressed down into the head whether fitting or removing. I don't have a press, so I oil the guides and use a BFH instead. Carefully...
As I said previously, use a 9/32" solid reamer after fitting them. Some guides particularly bronze can close slightly during fitment.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Kev in terms of the brass punch. Wouldnt the 14mm hole depth be critical. Is this what you mean by length. Nice to see the white table in the pics again.

Yes.
What is critical is the stick out of the guide above the head. Too low and oil goes down the exhaust guides, too high and the seals get damaged at full lift with 1.5 rockers. As you say it is 14mm using my punch and bush combination. (56-42) mm.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.