From what I've heard the Zeta Nagants were terrible, and that's very real AHP, I've saw the conversion done, but it required two guns. Anyway, why not use the Tanaka with the HPA, AFAIK, the Dboys isn't upgradable and to modify it to fit upgrade parts isn't fun from what I've heard.
There used to be someone in my local state boards that would sell kar98's. He would make a wood kar98 stock and fit bar10 internals into it. Did a wicked job at it too haven't seen him around in a long time though.