Boy, welcome to the rabbit hole. Where do I start?
It used to be modding involved Seiko cases only. I've built lots based on the SKX781 and there are plenty of people making parts for that. Many other people used other cases. Then the Prospex models came along (compressing the timeline a bit) like the SRP777 and the suppliers jumped all over that too. Then people such as yourself started wanting more cases with crowns at 3:00 instead of 4:00 and the suppliers started making custom cases like that. Now? You can get almost anything you want.
Almost all of them use the NH35 or NH36 movement. These are the unbranded version of the Seiko 4R36, otherwise identical. They're great movements, they (can) keep COSC time and they're durable. And they're cheap. $30 gets you another, there's no sense even repairing one.
So once you have your case and movement, you can mix and match to your heart's content. I did this for awhile and made two "Doxa" lookalikes out of SRP777 cases. They came out great. Here are two of them:
Both have stock Seiko SRP777 cases. Both have Yobokies dials and hands. The orange one has a bezel from Yobokies, the yellow one has a bezel called "Revenge One" from One Second Closer. The orange one has a domed crystal from Crystaltimes, the yellow one has a polished chapter ring from Yobokies and a flat AR coated crystal from Yobokies. Both have generic beads of rice bands available from eBay for cheap, and the endlinks (that fit the band perfectly and take a little shaping for the case) come from WatchGecko. Crowns and casebacks came from Crystaltimes and Dagaz.
This is just an example of what can be done. Get an inexpensive set of watch tools and go for it. If you screw up some hands trying to reset them, or if you scratch a dial, it's no big loss. Good luck!
https://usa.crystaltimes.net/product-category/cases/
https://www.watchgecko.com/metal-watch-straps/accessories-parts
https://usa.crystaltimes.net/product-category/srp-turtle-mod-parts/
https://one-second-closer.com/shop/the-revenge-one-polished-seiko-srp-new-turtle-mod-bezel-ntd-05-p/
https://calibercorner.com/seiko-caliber-nh36/
https://www.theyobokies.com/?mod=product&sec=item&task=detail&id=684
https://www.dagazwatch.com/apps/webstore/
https://suppaparts.com/Beads-of-Rice-bracelet.php