The guy I bought my 5.8 from told me his 4hp Mariner aux (which I didn't get) was ok for up to about 5 knots in calm conditions but that rapidly become not many knots at all into any kind of headwind or into any sea. So I have just ordered a 6hp Suzuki 4 stroke
for my aux which I hope will do the job.
Edited to say that just for info I found out that the Johnson and Suzuki engines in that size range are all made by Suzuki and the 4, 5 and 6hp 4 strokes are all basically the same engine just with a different carb, so the 6hp is quite a light engine about 26kg I think for the long shaft one. Got mine from Outboards Direct, Charles Oakley is a bugger to get hold of by email but a very very helpful chap when you eventually do!