There is a conversion calculator on here from Steve Debben ('debben1' i think) to take existing 630 sizes to 530 sizes. Dont end up with 15 45 90 if you see what i mean.
If its a unitrak output shaft it will have a large flat nut with inbuilt spreader washer. The locking mechanism for this is a large flat tab washer with splined internal.
The spline pattern is the same for the J output shaft just a different locking mechanism.
If you have access to a welder; get the tab washer blobbed onto the sprocket.
How many links are in the existing chain if there is one?
Using a different wheel but of the same rolling radius will make no difference to the gearing.
If in doubt order a chain a few links over and cut them off when you get the wheel set in the correct position.
Have just changed the J from 630 to 530.
It was 15 42 98 in 630 size
is now 17 47 114 in 530 size
That is not a direct conversion because of limitations with supply of sizes and the fact that i already had a 530 17 tooth front i wanted to use.
15 in 630 converts to 18 in 530 i think.
42 tooth rear in 630 becomes a 50.4 in 530.
and 98 links of 630 becomes 118 in 530 if what i wote is correct.
In any event my 17 47 114 could really be about 18 47 or 17 44 because its currently under-geared and though it picks up well it runs out of puff at about 115 - 120 mph.
Overall reduction ratio as standard for 15 / 47 is 2.733 The 17 47 gears i am using gives 2.764
Front sprocket was an off set one i had and the rear came direct from Talon in Somerset.
They also do offset sprockets periodically.
Debbens may have some in stock.
Debbens as well as others have chain by the length off a roll.
AL
I machined off the ends a fitted a pair of anodised bar end weights which fortunately just happened to be the same size.