Original grips are expensive and brittle as all hell. I'd maybe try to find aftermarket ones that look closer to the originals. But if it shoots fine with the ones that are on there, I don't know if I'd bother putting them on.
Can't help you with the value. I'd say around $1,100, but based on nothing but a WAG.
You have one of those guns that even though it appears to be ridden hard and put away wet it's still sexier than a hell of a lot of it's younger sisters. The actual value is probably $300 so I would expect you could get $500 easily enough. BTW I translate value as trade in value or the amount of cash you could easily get for it today. $1100 would IMO be a real stretch for a PPK in that condition. Nice ones sell for that all day.
Leave it like it is and enjoy it for what it is. Wood grips make for an even more enjoyable PPK 7.65 shooting experience.