I think there are a number of survival equipment service companies that ... may... still stock the " EP 104" Li-SO2 battery paks for that EPIRB and would do the replacement for large money.
Back in 2007, since I already had a brand new ACR GIPIRB CAT 1 & figured there was little to lose by opening up a discontinued Alden Satfind-406, I carefully unscrewed the case to find there is nothing at all complicated inside. Battery contacts were just simple 9volt-type snap-on male/female connector (though it's a 12V battery). Very simple and straightforward and rugged. I cleaned everything ( mostly o-ring case indentation) with clean cloth & closed back up.
After contacting the presently existing Alden Weather Systems Co. and considerable subsequent tracking, I eventually was able to buy an "EP-104" EPIRB battery replacement kit ( which includes the exact same battery pak the service companies use, and 2 (or 3?)correct case o-rings, plus a vial of dielectric silicone grease.
The kit invoice( $185 + $9 shipping) was from MORCOM INTERNATIONAL, Inc, parent of Alden Weather Systems and it really came from SAFT America who makes or distributes the batteries and were very helpful. Presently, SAFT America no longer seems to list the EP-104 on their website but I wouldn't be too surprised if they can provide it or a better than equivalent setup pretty easily if you were to choose the do it yourself method. But maybe not?
Once I got the kit, it took me about 6-10 minutes to reopen the Satfind , replace the battery and fasten the connnector with a little grease , remove old case o-rings and grease new ones and carefully install & double check them, then carefully put the case back on taking care that o-rings were safe. I had chosen a very low humidity day inside the house.
No harder than replacing portable radio batteries but I was taking extra care due to it's intended purpose. If timed I could have done it in probably around 3 minutes or less after doing once. Tested fine in the normal fashion as per the label on the EPIRB case. The kit also had the new Expiration Date sticker and clear protective over-sticker.
I easily replaced the commonly available Cat I hydrostatic release separately and of course kept registration info updated and that sticker renewed regularly, so it's back on the boat and good thru Oct 2012. http://www.cmhammar.com/products/h20-for-epirbs
After the replacement & test I found no reason at all to trust that Satfind any less than a new one, but I do also have that GPIRB & a good PLB. I will note I was told a bunch of definite BS by a couple of authorized service companies I checked with 1st before talking to Alden and SAFT America. .
No bashing please, not making any recommendation here.