I have an old Siamese mauser which was converted by ??? to .45-70. It looks okay, but doesn't chamber rounds from the magazine worth a damn. What I'm wanting to know is what mods are necessary to feed from the magazine. I'm pretty sure its not a Navy Arms rifle, since it doesn't say Navy Arms anywhere on it. I think that whoever built it maybe didn't machine the receiver enough (or at all). The only parts that I can see any marks on is the sides of the follower. Maybe it was ground too much? I don't know. I've also thought about replacing the mag spring, since if its weak, it may be causing the rounds to dive some as they're going into the chamber. Right now, I'm not even sure if the mag spring is the right one, or if a generic one for a 98 mauser is the same or not.

Anyone?

As of a decade ago, E. R. Shaw was still doing conversions of the Siamese Mausers to .45-70. IIRC, it involves both magazine box and receiver rails. Dunno if they are still doing those conversions, but they are at least familiar with the required work, and may be able to tweak your rifle to work properly.
Navy Arms was the big distributor of converted rifles, but there were quite a few smaller makers, and those conversions were not always done correctly. If your rails aren't modified, that may be the cause (or one of the causes) of your feeding issues.
FWIW, George Nonte did an article on the conversion in Rifle magazine back in 1970. A digital back issue is still available:  Rifle No. 12