The bracelets of IWC diver's watches have not had screws since 1982 – but that's only by the way.
In order to be able to help properly, further information would be helpful. The pins on the bracelets of the three-hand Aquatimer references 3536, 3538 and 3548 and their chronograph sisters have 9(!) different lengths due to the tapered clasp. The later references 3568, 3290 and 3288, on the other hand, only have one length. For the pin, I would either ask the IWC dealer around the corner or contact IWC's concierge service, which in my experience is very friendly and competent. Occasionally, strap links are also offered at C24 and in the bay, maybe have a look there as well - but be careful, they often ask moon prices.
Strap alternatives for all IWC diver's watches with the exception of references (1)812, 1816, 1822, 3231 and limited 3290 / 3288 are only available from IWC itself due to the proprietary strap connection and are correspondingly expensive. For the references 3536, 3538 and 3548, in addition to the bracelet and the rubber strap, there is also a Velcro strap with matching end links (titanium: reference IWA04920, stainless steel: reference IWA04925). This also allows NATO straps (20mm) to be used. The Velcro straps are still available from IWC, cost 175.00 euros here in Germany iirc and are of catastrophic quality, there are a few sellers on eBay who also sell these straps - but at double or triple the price.
For the reference 3568 (and their corresponding chronograph sisters),
simolau offers rubber straps in various colors (red, yellow, orange, blue, white, black) in the bay, the quality of which is said to be very good.
On the (more or less) current Aquatimers of references 3290 and 3288, all possible straps in 22mm width can be fitted if you have the courage to cut through the fixed bridge and replace it with a suitable spring bar. For my part, however, I will do the devil here, because in my opinion the original IWC bracelets and straps are among the best available.