Generally speaking, Y&B in combination with a wheel adapter should be able to steer this boat. For the negative transom an extra mounting tube is probably also needed.
Fitting self steering to my boat
For a boat with a transom hung rudder I recommend Y&B standard with an extra mounting tube.
This system has a mounting kit included that together with the extra mounting tube allows for mounting around and behind the rudder.
For fitting to smaller boats Y&B can be ordered with a short pendulum rudder, which is a free option.
The system that is advised for a centre motor is Mr. Vane, but that is for a centrally mounted outboard engine.
For a central swim ladder on a scoop Y&B standard + an extra mounting tube may work, but that depends on how the ladder deploys.
The blue boat on the Y&B product page has a telescopic slide out ladder:
The Mr. Vane self steering system was designed to be mounted below and in front of an outboard engine fitted on a specific type of outboard bracket.
For this purpose, the vane mast can be fitted to the vane base off centre, allowing the engine to be lifted up (outboard bracket) and tilted (engine mechanism).
This can be seen in the Mister Vee brochure available from the download section.
The mini 650 type of boat has the ability to plane.
This allows for big changes in speed with small wind speed increase, and that gives a big change in apparent wind direction.
This leads to a zig-zag course which is very unsatisfying.
This is why a windvane self steering system is not recommended for this type of boat.
The mounting kit that is included with Y&B standard does allow for the kind of mounting a sugar scoop requires. From the pictures on the internet it is unclear if you would need just 1 extra mounting tube or more, the scoop seems to extend quite a bit.
If your boat has wheel steering, you would also need a wheel adapter. But if the boat has an option to connect the self steering system to an emergency tiller, that would be easier to realize and easier to use.
The emergency tiller would need to be about 30 cm/1' long and can point backwards if that makes things easier.
For the transom described in the question the single tube that is included with Y&B standard should be enough.
The Y&B vane base is about 200mm/8" long so the pendulum rudder will well behind the rear of the transom.
As long as the rudder does not stick out behind the transom, that distance should suffice.
If you like to examine these kinds of questions before contacting us, there is a drawing with dimensions in the download section of the website.
The best advice would be to remove the ladder, then fit self steering, then find a new solution for the boarding the boat, either by relocating the old ladder or fitting a new one.
Trying to combine self steering with an existing ladder almost always means much more work to fit, less satisfying performance or use of the self steering system and more difficult use of the ladder.
The vane mast can be shortened by about 70 cm to suit a specific situation.
But just to state the obvious, the windvane still needs to get wind.
To reduce the height of the vane mast, the pushrod that runs inside the vane mast needs to be shortened. It comes in two pieces, to be glued by the customer. Best to leave that to be the last task, after assembly and installation.