Would a magnum length cartridge fit in to a large ring Peruvian Mauser action without significant modification?
You really need to be more specific. Which magnum round are you talking about.
Many magnum rounds were kept to a certain length on purpose so that they would fit into standard length actions.
Also, which 98 action built for Peru do you have? The 1909 and 1935 are different. The 1909 has a longer receiver ring if I recall.
Whatever you decide, you are looking at a lot of expensive work. Darn good base actions though.
Take a look at one of our banner sponsors called Tradex. They have lots of purpose built sport mausers for sale. Especially if you're looking for a donor.
Most of the work needed to make a nice clean sporter has already been done.
If you're looking to convert to one of the extra long cartridges, like the new ultramags or one of the older long cartridges, it can be done but whether it will be worth it to you in the end, is a personal thing only you can answer.
If the cartridge you want to go to is already being produced in a commercial rifle, it would probably be cheaper to buy a new one in the end.