I have a Siamese mauser which I originally bought with the idea of one day converting to 45-70. It still hasn't happened.
There are more than a few problems. The bolt face has to be opened up to accommodate the rim size. Not too big a hassle, but requires a good machinist. Frequently, the feed rails in the action may need to be tuned to allow the cartridge to feed but not so much as to cause misfeeds. Finding a smith experienced with the Siamese action may be an issue. Because of the way the magazine was designed, most Siamese mausers are missing one or all of the following: follower, original follower spring, floorplate. These are unavailable worldwide. The spring can be substituted with a Springfield or similar but I'm not convinced its a good match. A Japanese Type 38 floorplate can be modified to suit the Siamese Mauser, Hoosier gunworks have instructions. The follower is a big problem. A standard mauser follower is a poor match as are most of the recommendations commonly guessed at on the net.
Most of the minor parts are also no longer available. They are similar to standard mauser parts but just different enough to not fit. Likewise, some threads are standard mauser threads (Whitworth form) however others are obscure Japanese threads from a per-standardisation era, neither metric nor inch based.
Gun Parts Corp in the USA have some parts, but in 2018 they stopped international sales. I don't know if they have since resumed.
A good machinist can overcome all of these issues, but just be aware of what the project may involve.