chance of a lifetime "King's Ametyst"

To add to what Tinski has posted –
She is cutter-rigged with an inner and an outer forestay. In those cases the inner often is demountable at deck level so that it can be brought back to the mast to be kept out of the way. Also looks as if she had twin backstays, possibly low down but then joining further up to form a single backstay (good because it spreads the load on the transom.)

Mainsail track - usually an alloy of bronze on older boats, essentially a flat length of metal with lips forming a groove into which the sliders on the luff and foot of the main fit to allow it to be raised /lowered. Screwed at about 6 inch centres. For halyards I'd use wire spliced to rope, that way there is more room inside the mast. Don't forget space for electrics (e.g. steaming light.)

Boom gooseneck fitting – I’d go for a basic fitting and would not bother with roller reefing; slab reefing is more effective because it gives a better shape to the sail and does not cause kicking-strap problems.

Brackets for spreaders - not really load-bearing, they mainly just keep the spreaders in place.

She has at least inner and outer shrouds, the inners probably meeting the mast just below the crosstrees. Possibly two outers?

I would be inclined to have a couple of sheaves, one for main halyard, one for jib. The block for the topping lift need not be too strong as it carries only the weight of the boom. Consider if you will fly a spinnaker, you will need a block for that also. And one for the jib. Are you going to consider a furling jib?

One good winch centrally mounted ahead of the cockpit on the cabintop with a row of jam-cleats in front of it and all halyards leading though it should work. Be very careful in getting the angles correct where the halyards come out of the mast.

I know the ‘birds mouth’ is just a model but I’d source the closest-grained timber I could find for the mast.

Is there any way you could get to a yacht club/marina to have a really good look at some older boats to see this stuff in real life? Most masts are ‘up’ at this stage, but come September many will be on the flat and readily inspectable.

Below is a modern masthead on a small sloop, showing (typically) a bar through the top to carry pins and sheaves for forestay, backstay, halyards, etc. Not exactly what you are looking for but it should give you an idea...
Great to see the progress you are making, keep up the good work.:)

First of all I’ve never built a mast, just looked at lots of them on various boats down the years, so I’m no construction expert.

Groove in boom/mast. – there are two types, one which allows the boltrope on the sail to be fed through it, the other takes ‘slug sliders’ that fit into the groove and are attached to the luff or foot. The former IMO is OK for the foot, but if you are going to recess the luff I would use slugs. When the sail is fed into the groove it inevitably is on racing yachts as it give a marginally better airflow over the leading edge of the sail.

Track - screwed on externally, but the back of the mast could be routed to recess it. Important to have as few joints as possible to prevent sliders from catching. Obviously screwed as short centres and countersunk. Make sure that the screws and the track are the same metal (avoid electrolysis.)

Inside/outside halyards – on an old boat I sailed on as a kid had a hollow mast and the halyards went up inside it. They were SS wire spliced to rope – the wire gave strength and did not take up much room inside the mast. If you are going to spend time sleeping aboard having the halyards inside is much quieter, otherwise you will have to tie them out.

Scarfing – instinct tells me that you should avoid it, as I feel it would be a weak point. I’d use different lengths of timber to minimise weak-point joints.

Are you near any of the bigger yacht clubs where a boatman could give you a look at a few examples?