I scaled the original SS to a 24mm engine, and ended up drafting and printing all my own decals. You can buy blank waterslide stock and print the decals with your inkjet printer (I used a laser printer and adhesive label stock).
Search for my Solar Sailer thread to see how mine turned out, and good luck with yours!
I have my 1.68x upscale Solar Sailer II completely built and painted, waiting on the decals as I type this.
Astronboy is putting them together for me (long story, but I know he's got 'em and I know he has them in the right size!)
I'm assuming that you are using BT60's for the "main" tubes and either a BT55 or a BT52 for the central tube?
That's a convenient size because you can use a readily available nose cone (the "big bertha" or "baby bertha" cone, available in plastic from Estes or balsa from anyone who sells cones) for the nose cone.
If you simply scaled up the central tube from a BT20 to a BT50 and made it 24mm powered, you could use a BT55 for the lower and upper tubes, but a rounded BT55 nose cone isn't as readily available.
Anyway, I'm getting all over the place here.
Long story short: I have an original kit, and can give you all of the original tube lengths if you want them (because the plans on JimZ's have those crappy long part numbers that are meaningless and don't cross reference to anything. You can derive the length of the lower BT50 from the instructions themselves by reading carefully, but not the upper....)
I can also give you the scaled up dimensions.
Also, Astronboy has the decals all made up and can print them to the scale you want. If you're building it BT60 based with a BT55 or BT52 central tube, he'll have them made up and ready to just print-and-ship.
(did I mention that I *love* Astronboy's work and fully endorse it?)
Good luck, and take great patience when assembling those 3 - piece main fins. Depending on how picky you are about your builds, and what diameter dowel you use vs. the thickness of your fins, this will be the most difficult part of your build.
Having a woodworking shop available, I cheated. I have a set of wooden buttons with shallow dados on one edge. The dado offsets the dowels just the right amount, so a pair of those with a clothespin hold everything in place.
Also, I cut small holes for the dowel tips - i.e. the dowels are TTW as all three on my original SS came loose. The fins are TTW also.
Consider NOT putting an engine block in, so you can mount D, E, or F engines. Mine has a block, so Es stick out the back an extra inch.
Thanx for the decal advice, I might in the near future be ordering some, so i will be in touch. I too am cheating a little, having an uncle living 20 min. away with a full machine shop, but hey, if you have it use it, I say. When done mine will measure about 6 ft. tall with 2.5 diam. airframes, and 1.5 diam. middle section tube, not exactly to scale but will still pretty cool.