I got a 1972 Johnson 20 which claims to be a 25.... (Was bought in my younger days before I realised how easy it was to swap the lids around.
It's Longshaft, but I could try to source the short drive shaft & water tube to "shrink" it - that vintage of Johnsorude they just added a spacer to the leg.
If it's not too old for your liking, let me know and I'll see if I can track down a short driveshaft etc. There are photos in here http://www.rib.net/forum/f10/1972-jo...ban-49078.html
when I thought it was a 25. Embarrassed thanks to Terry for the comparison with a real 25.
(...and it's no longer lying Oban)