My '68 Raleigh project came with cotters, I tried EVERYTHING I could find on the net and at the shop I was working at at the time, one side invariably came loose every 2 or 3 rides. Replacing the cotter made it even worse...since you can't *get* high quality cotters anymore, they're only available in cheap-ass pot metal flavor.

I'm not sure if Japanese frames use their own standard for BB size/threading, but I'm guessing not...just standard 68mm English. I replaced the old unsealed cottered nonsense with a used Tiagra set someone was throwing away (but nevertheless in great working condition). You'll wanna double check BB size with some calipers to be sure, but there probably won't be an issue upgrading.