Can't help on the kits to buy, I'm way out of the loop. But you can covert a two handed axe fairly easy (minus pinning perhaps). Especially if you already have axes. (triply if you happen to have the OG berzerker axes without hands.)
Otherwise,
zinge industries sells flexible chain teeth, they are a little bit on the large side, but for a heavy axe that works perfectly.
It has the round attachment points too like the
GW ones:
That is one of the newer axes and the chain is as wide as that on the links, hence the shroud to hide it a little.
It's also possible to shave of the teeth of an existing chain weapon, but you have basically zero room for failure.
If you cut it well though it's flexible as well (to a point) and can stick with plastic glue.
This axe head is all styrene and the teeth were peeled from a chainsword. it's not as tough as it sounds, detailing the resulting weapon on the other hand can be super fiddly.