Have to agree with Pete, whilst the blue bands are good engines, they are getting on abit and it probably wouldn't be economical to replace the power head. I am also a fan of the 40HP Mariner and as Pete says they are quite indestructable, I believe they are of Yamaha origins, and would probably be my choice for an SR4. And yes I do have one for sale http://www.boatsandoutboards.co.uk/view/F87628
Although the one Pete has pointed out seems a good deal. Incidently, the 40HP Mariner I have is smoother at idle than my 40HP Suzuki, both of which are 2cyliner 2 strokes, 40HPs.