In that case, you may get lucky and find a thicker aftermarket tension spring, there are a few on the bay, here's one:
http://www.ebay.com/itm/BEZEL-INSERT-TENSION-FLAT-SPRING-FOR-ROLEX-SUBMARINER-1680-5508-5512-5513-5517-/160907267371?hash=item2576d20d2b:g:LEAAAOSw4A5YpNfR
Try it in situ with the crystal you may find it becomes tighter. Also, there should be a tension spring between the bezel and retaining ring that you can bend slightly to cause friction.
There are also solid midlink bracelets available without the line on the last link. WSO has them, you will just need to swop out the fliplock with the rep version as the WSO version is unmarked. WSO also sell the gen spec 580 endlinks and 2mm spring bars with rounded ends: http://m.ebay.com/itm/SOLID-OYSTER-BAND-BRACELET-FOR-ROLEX-SUBMARINER-FAT-SPRING-BAR-20MM-93150-F-LOCK-/310586969327?nav=WATCHING_ACTIVE
The newer Cartel 5513 midcase is actually what they use for the 1665, hence the extra thickness. You can check out some comparison shots in my 5513 build thread: https://rwg.cc/topic/181702-the-updated-cartel-5513/#comment-1447801 Be wary of the 1680 cases you may find it requires a slightly larger dial 26.5 - 27mm rather than the 26mm of the 5513. Here are my Cartel builds, the newer version on the wrist, older in the hand.
MQ is worth checking out for a high-end dial: http://vintagewatchesmq.com/minhquy/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=800&category_id=146&option=com_virtuemart&Itemid=1 There are no accurate inserts. There is one from Helanrou with serif numbers, however they are too small and the quality of the insert material itself pretty poor. Here's mine with an ETA 2836 movement next to s gen. They dial was from Ingod44, unfortunately no longer in business.
No the crystal hasn't been modded. As previously mentioned it's a Clark T21. The top lip of the bezel has been sanded slightly so there's a small ridge around the top now. The watch is comparable in size to a 1016 and wears well.