I'm sorry, but your description is still not complete as what you describe could be one of a couple rifles and in the absence of some pix or a better description it is difficult to tell what you have. Does the magazine extend below the stock? Does the rifle have a two-piece stock? Does the rifle have a straight-pull action?

One thing you should realize is that conversions of even easy-to-convert milsurp guns {like M98-action Mausers} typically cost more when it is said and done than the cost of a new gun in the calibers you describe. You seem to have the notion that the gun is highly valuable. In a cobbled-on sporterized condition you describe a milsurp rifle has little value compared to an original-condition rifle.

Sometime post some pic's if you can. In the meantime my vote is to leave it alone and buy a factory gun chambered in the calibers of your interest.