I don't know the production numbers, but as far as I know they're pretty rare. I had a 71 SL 338 and 71 SS-433, both were drop dead reliable and we put thousands of trouble free miles on both. The 70 and 71's were still belt drive oil pumps, so after all of these years maybe you should look for a new belt? We never had a problem with the belts ever, but after 30 some years its probably not a bad idea. The SS 396 was rated at 36 HP. Sweet little sled save for the bogies, but a later model slide suspension will bolt in with some work and drilling a couple of new holes. A very good idea as the single sprocket tracks are hard to find in decent shape and are prone to breakage. Thats the one area where we did have problems, both sleds spit the track out the back. The involute drivers would have to be installed as well, but should nearly be a direct bolt in to my knowledge. Where the spindles come through the belley pan they crack the welds out, but it doesn't matter, we'd weld them and they'd crack over and over, so finally we just left them cracked and they never caused any other problems.
An Exciter 440 will bolt right in and give you a gear driven oil pump, reeds, and CDI ignition in the process, and bump you right up to 48 HP too! I talked with Aaen and they're willing to sell a pipe in pieces to make it fit for the application which is good for another 14 HP! But I don't know if you'd wanna mess with a factory piped sled. Best of luck with the old girl. G.