Hi Andy. While I'm not familiar with your particular motor, I've made a simply tiller extender for my '66 20hp Johnson by using a piece of thin-walled PVC. You simply cut 3-4 approximately 2''-3'' (4-6cm) long in one end, slide it over the stock tiller and hold it in place with a small pipe clamp. I've used mine for years without issue. Best of all, it pops right off and only cost a few bucks (quid) and it lets me sit/stand and move my weight around while underway as needed. Of course, there are fancy aluminum and carbon fiber versions, but that's probable overkill. Here's a pic of mine in situ.
View attachment 204903View attachment 204904
View attachment 204902 (fancy schmancy versions)