The maker's recommendation is to move the footing (four metal studs) for the existing straight engine mount, 17cm to the side. Where I live there are far more light metal fabricators than FRP shops, and I'm not strongly tempted by the thought of cutting into the plastic monocoque just to end up with an assymetric motor position. The existing aluminum motor mount is a straight shot in 5cm x 3cm rectangular-section aluminium. My measurements today suggest I've got a gap at full helm that a 40mm or 45mm section will comfortably get through without having to take too long a path sideways. The system is still twin-rudder - the previous pintles being replaced by gudgeons that use those same mountings.
The four screws are for mounting a rowlock for a sculling oar - removed by the previous owner, I guess. Location: Hampshire UK I've checked out the two designs of rudder attachment on that boat so I now understand the change.
Yes, I'd forgotten about that option - easier to draw and easier to build: and I realise now I haven't taken the measurements for it. Overall, I'm planning to keep the mounting point in the same position relative to the hull. Equally a little light heat (local annealing) on the 'hinge' area won't hurt when persuading it. Location: Pacific Yes, another aspect is sail balance - we're fine in 15 knots, but at 25 it'd take a well-oiled crew and a braver man than me to get out the gennaker. In the Mark 1, the owner set up the toe-in post-delivery (two-part control arm with one pole inside another; electrical tape) and it's the same with the new setup, this time by rotating the control arm on each side on the threaded attachment to the tiller.
Looking at the second sketch again, I see I do have the measurements for the overhang of the tiller arm at dead-ahead: it's 28mm out to the pivot plus 214mm to the outboard end = 242mm. If the motor bracket is an L shape rather than flat plate + ply block, you can come up much more vertically with the rectangular tube and it will still give enough support.

Another solution is a topside brace that drops in like a rudder on pins but may need wing nuts to tighten.
It may also be possible to fold the rear mount plate if it is thin enough but be aware of the resulting radius. Location: Pacific Some more focussed CAD drawing and I now have this (attached). Bringing the tube up inboard of the control arms doesn't give a better angle out to the engine mounting plate. Location: Hampshire UK Personally, looking at the side elevation on the right hand side, I would have put the L bracket the other way up.
I do want to adapt to what stock extrusions the fabricators have, though, so I'm not thinking anything's final yet. The drawings are actually 3D models with rough rendering only, modelled in Rhino with render using Flamingo plug in. The horizontal strut is a 3mm, 70 x 30 extrusion; the standing strut is cut from 3mm sheet and bent into a U, with a rear side cut, bent and welded in. When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Hard to make out the detail from available shots, but assume its hunky dory if they haven't exploded. If you have a good fabricator he may also have access to an anodising facility, 20-25 microns would be enough, though it is not strictly necessary to anodise.
Very crude, lazy effort on my part but the 3D model is accurate and real world size to your application. Yes, you can curve this type of section with rollers and it is possible if the fabricator has a suitably large set. That way the box section is sealed inside differently and acts as a gusset support vertically (transom side) for you.

I would solve the water trap problem with a couple of limber holes (as in plan view) and also ensure slight local raising of the ply so water does drain and not rot it. The chop into the box for the plate keeps good integrity too, I can see why it has got there. Too thin (too slim a section), too narrow front to back, not enough glass (my own gut feel) along with too light a foam or core material in the head area. You don't want to see little stress fractures, if you do, then two pieces is a better solution.
I suspect the V notch or butted 2 section solution will be easier and OK, even acceptable visually given the constraints of the new tiller arrangement. It is a more awkward chop in the box section, but doable and the weld area is longer but working over a better area for you. Use the same guage as you already have for the end plates especially for the keyholed part. It is also possible the rudders slipped out of being parallel or out of any toe in as set up by the yard, resulting in extra stress.
If in doubt a small gusset could be welded inside the V but I doubt it will be necessary unless you up the motor weight. Material could be 6082 T4 (one of the stiffest alloys you can get off the shelf) preferably treated after welding to restore tensile strength.
This can be quite important as the treatment can double the tensile strength of the same alloy.

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