99% of the pro mechanics I know out there use brake fluid to lube the piston seal, not grease. Grease is for the slide pins external to the caliper and won't contaminate the brake fluid - which your bike doesn't have because it uses that swing caliper bracket. IIRC the bracket has a zerk fitting on it so whatever chassis grease you have is fine since it doesn't get hot like the caliper.Really? Usually all I find is generic caliper grease.
I'm talking about what lube to use for the piston/seal.
you don't need to buy a "new" caliper unless you want to do a brake upgrade (and if you do, I know someone that is the US beringer dealer with a kit for cb750's but it's pricy). Just buy a new piston (phenolic is best) and seal and rebuild your old caliper. Almost all cb750s have a sticking caliper from sitting because the chrome steel one rusts and hangs up in the bore.
While you are in there futzing with the brakes - order stainless braided lines - way better than stock rubber in that they won't expand and get spongy.