Medium animal, Str 18-20, climb speed, slam/slam/bite attack pattern?
I'd go for Strength 20. There hasn't been much study of how strong chimps are, but the little there have been showed them to be at least four times stronger than a human. Since a +10 Strength score can carry 4 times as much weight, that suggests a strength of 20+.
Come to think of it, the SRD Ape's ability scores look pretty close:
Str 21, Dex 15, Con 14, Int 2, Wis 12, Cha 7
Since it's a size smaller we could drop the Con to 12. The intelligence needs to be increased a bit - how about Int 6, the middle of AD&D Low Intelligence range of Int 5-7?
Str 21, Dex 15, Con 12, Int 6, Wis 12, Cha 7
I agree with 2 slams and a bite, but would make the bite the primary attack. If I remember correctly chimps, like most primates, do most of their damage with their teeth when fighting.
Two Hit Dice seems the way to go.
Medium Animal
Hit Dice 2d8
+2 (
11 hp)
Speed: 30 ft, climb 20 ft (The SRD Ape's climb 30 is a bit too high for a "realistic" Chimp)
Attack: Bite 1d4
+5, Slam 1d4
+5
Full attack: Bite 1d4
+5 and 2 slams 1d2
+2
Special Qualities: Low-light vision, poor swimmer, scent
Advancement: 3-4 HD (Medium) (The SRD Ape goes to double HD, same size)