I'm doing a 9.9 the other way round.

Hi,
Your Shetland Suntrip is a neat boat. Standup piloting, a cubby to warm up in and with Johnson 50hp; you'll be a flying at a high rate of speed to that fishing hole. NICE. From the picture I was able to pull up on the internet; the transum height looks tobe just about standard, ~15-16"; is it bigger than that?; that would require a longer shaft. I know you stated the need and all but; looks tobe OK.
I'm converting my Johnson 9.9 Sailmaster longshaft to a standard drop. There's a chance that the unit has 15hp (from the factory parts book) even though 9.9 is all over the engine cowling. Maybe dreaming. At any rate I have the original old J28hp that came with my MirroCraft fishing boat, I'm restoring; and the 9.9 will do till that happens. I'm retired and don't need toget anywhere that fast.
Converting driveshaft length (long to short or vise versa) is new territory for me and ~$270 in parts (drive shaft, water tube / gromet and shift rod); is a bit of a lump. Something about equipment dignaty that drives me. Best local parts counter advise is to look to purchase the unit I need; but I am not detired. Still keeping the shift sattle and drive pinion gear aligned while changing out components seems alittle iffy and taking the gear box apart to make it happen possibly bothers me. But things could go with out a hitch.
There's a guy that published on the internet about shaft converging of a 9.9 (at least). I guess they're all very similar. You can read and get the best
info on the job (I know of). See: Leeroy's rablings shaft conversion .That right; a real boat guy/engineer who took the time to write the specifics; what a saving grace.
Good luck, let me know how it goes. I'll update you as to how it goes, for me. Smooth sailing,, I hope.

Not sure if thats offered by Johnson

Found no referance to a long shaft 50hp engine.(?).
Boats.net seems to have a full list of parts; including both
long and standard shaft; from my
experience with the 9.9hp model. They've got the price
nailed amoung there competitors. Give a fast phone call to
Boats.net people; I'm sure they can give you the low-down.
My J9.9selcom makes a great kicker for sailboats and
large transum yatches but your boat should have a standard
height transum (~15"-16"); at a guess.
If so the motor you got should work fine. Is my thinking; not
really haveing any experience with that particular motor!
The deeper prop gives you a better bite in the water, 'I've herd it
said',but not necessary; and then you got to watch it in the shallows.