I think 'stock' involves buying (twice the cost+) and changing (more labour = more cost) a whole new IMS shaft though, don't think Porsche sell just the bearings.
Looks like the retrofit does have a warranty, 2 year / 24000 miles limited warranty. See bottom of
IMS Retrofit – LN Engineering Genuine IMS Bearing Upgrade Kits for Porsche ® Engines
To be fair though, all the alternative aftermarket replacements have the same warranty clauses, they'll only ever cover replacing their product if it dies, not the engine or anything else it takes with it... it's just not much use having a replacement bearing and no working engine to put it in
