/2 Sidecar combination?

[Flx48]

As Dave suggests, you probably have the standard 25/8 gears in the rear, and a solo "S" stamped trans.

But if you wish to confirm what ratios you actually have (afterall 60 years is a long time) without ​disassembly, the most common method is by feel through the driveshaft rubber boot.

For the rearend, place the bike on the centerstand, so the wheel may be rotated.
Palpate the head of one of the four bolts that connect the driveshaft to the trans output flange under the rubber boot, and simultaneously note the wheel position.
The goal is to rotate the wheel one complete revolution, while counting the number of driveshaft revolutions.

In the case of 25/8 gears, the driveshaft will rotate 3.13 revolutions to turn the wheel 1 complete revolution; so that's 3 complete revolutions of the driveshaft plus about an 1/8 of a turn.
For 27/8 gears, (US models) that's 3.38 revolutions, or 3 complete revolutions plus a little more than a quarter turn.
27/7 gears (3.86) would be 3 revolutions plus around 7/8 of a turn.
And 26/6 gears (4.33) would be 4 revolutions plus a little more than a quarter turn.

Finding which gearset is in the trans is a similar exercise, first gear being the simplest. (solo 4.17, s/c 5.33)
Rotate the input shaft and note how many turns gets one revolution from the output shaft.
This requires turning the crank over a number of times, so pull the plugs.
Either ease the motor over with the kickstart while on the flywheel counting the number of times the timing marks pass, or put a wrench on the nose of the generator, and count the number of turns from there.

As Dave said, some folks are happy running a s/c with the standard solo gearing.
What suits you is determined by your terrain, (flat/hilly/mountainous) the sidecar chosen, (light/medium/heavy) and your payload, (just you/maybe your little grandchild/or maybe your 350lb MIL) and what you determine is an outfit that is tractable enough for you.

If mostly driven in the FL flatlands, maybe sometimes the piedmont of the neighboring states, and lightly loaded, I might choose the rear from a US model; it is a just slightly lower ratio than standard, much more readily and reasonably available than either of the s/c gearsets, and probably a matching speedo would be available as well, if that's important to you.
Best-
George