I have looked into it my self, and it doesn’t seem as expensive as I thought. I think you need a rotary steering system with a cable, and the motor has the cylinder built in (between the black caps), a bracket to to bolt onto the place where the tiller was , a connecting rod, for this, a steering wheel and the control box with cables, which I am having trouble finding at a good price. An example of the steering system is
https://www.ebay.ca/itm/SS13713-Rot...ispr=1&hash=item2cf08eb615:g:TWUAAOSwmUhdOm0l
or you could go for a more expensive sea star system
(I have a 2004 bf25d, that still runs better than a new Yamaha)