I'd say to "cannibalize" code and art. I'll try to play devil's advocate here.
I can't say nothing if the developer has made a game entirely for free, nor if he had put up a donation drive and he's really working to sastify his funders, nor if he's having some problems but at least he notify his followers. Yet the developer not had the time to write a 3-row comments informing everyone that he's not working on the game but he's still interested on continuing it (or at least he is kinda at a dead point now, but at least knowing he's still alive), but he surely had the time to make the donation page disappear.
Hartista (time ago) said that, even if the donation feature was blocked because of the many features corrently under developing, you could have retracted the money at any time, in case you felt down to donating at any time. Yet, i think around... 2 weeks ago? The donation page was already blank, and no explanation was left from it.
The developer growed tired of making the game, but instead of admitting the abadonment of the game to everyone (as well as refunding the donation money) he just ran out with it.
That's my version, if he had caught in some sort of accident (like, other family stuff, or been cut-out from the internet due to its ISP/router, or for whatever reason he couldn't touch the PC) then not only no news or comments were spotted from him, but the donation page could also be left there as is. (He) Not being interested anymore in his game, why not taking what has made?
Maybe i may look a little harsh, but nobody considered this: will the new group/collab accept money or not? We don't necessarily need artists working at this for a relevant amount of time, so even small work from everyone can help make a big piece.
If no donation is accepted, wouldn't that count as "share the work but credit the original author and do not gain profit from it"? I see nothing bad here, considering that the whole donation stuff is closed, the only thing we can "steal and walk away" with is, code and graphics, but if we cite him for his work then we won't technically "steal" right?
I personally hope for the new group to be large in number as possible, counting the whole mess of the missing animations. Or at least, under my viewpoint, having a large group of OC animations (or even new monsters) can be a great way to relaunch the game, as well as attracting more people to play it (or even developing for it). Plot and game mechanics are great, but can be left as is, there are few critical freatures to fix off. These are my observations in a (very) resticted way, but i don't want to make unnecessary long WoTs and eventually go offtopic.
You can use decompilers to also take away at least the graphic, they not always work very well (sometimes they split the body to multiple istances like head/eyes/legs/arms etc), but you can rig them back, if you are good at using flash CS. This is useful if you aren't a good artist, having some knowledge in flash you can help with the project anyway.
You can also extract code and scrips but i haven't tested it personally.
Instead, making a new game from scratch would be also great (make stuff from anew istead of fixing broken stuff) but that also means getting new animations. I hope it won't (only) use the recycled sprites used in decades projects involving Krystal (Thus making Breeding Krystal: the game

) and no OCs or stuff from other projects. But i think this is the best path if you want to follow the "clean" way, and who knows, it might eventually be a great game.
Just... we should also discuss to whoever coordinate whatever the decompiled (or the new) project, and PLEASE, don't put someone to the lead that is like (i like monster/fetish X but i dislike monster/fetish Y, so won't put that in the game). The more stuff the better.