Personally, if I had a rifle in 43 Mauser, I'd want to load for it and shoot it even if just at a range but hunting with it if it works well enough would be a plus. I've often bought new rifles in different calibres just as an excuse to pickup more bullet molds. Being who I am, I'd also want to try black powder loads with it for fun and the experience.
-Slug the bore, find bullets of a suitable diameter (.001" to .002" over groove diameter). See if a heavy 45 pistol mold and custom sizer would work. Order a custom mold if necessary to cast my own. Could also try paper-patching some 44 mag up; would need thick paper as you're increasing at least .015". I normally PP .450" up to .460" for my 45-70's using "college ruled" 3-hole binder paper (.003" thick, wrapped twice, 2 layers per side, so .012" of paper but wrapping compresses it when wet so comes out to .010" to .011"). Computer printer paper might work; it's around .004" thick.
-Order the Lee dies to save some money if I'm not planning on shooting the rifle a lot. If making it one of my main shooters I would consider investing in the nicer RCBS dies but that's unlikely unless I'm in love with the rifle.
-Find some data, load, shoot, repeat.