Only FP's I have been able to find in the 7mm is Speer 130's. Just got 3 boxes form Natchez last week. Other than that, just a new barrel. I still had 12 boxes of Fed. factory ammo I was using and buying more when ever I found it. Hornady dropped their 139gr. FP, and have not seen any Seirra 120 FP's. Speer must have just made another run of the 130's, no one had any until I called Natchez 2 weeks ago. I use Nosler BT's in my 2 Single Shots, save the rest for the 94 Winchester. DP
Just the barrel change will put you in the running, unless you want to change the magazine configuration of course. And you'll have to make whatever machine work will be needed to make the magazine configuration work with the new barrel, i.e. dovetail cut for the tenon if half mag, or drill the hole for the mag tube plug screw if full stock. You'll want to have the replacement barrel about the same OD as the old one so it works with the barrel band if a full stock.
Sierra should still make a bullet for the little 7, and one can always make up a quick jig to hold a 139 gr. spitzer and grind the tip down with a belt sander. Nosler still make their slug for the Waters? The Speers are decent bullets in the Waters, works well on game.
From using a Waters in the past,250 yards wasn't that hard to do, but of course it depends on how you set up, iron sights will give a somewhat shorter zero with a 3" high hold at 100 yards because they are closer to the center of the bore. If you scope it, the higher sight line acts like it "flattens" out the trajectory a bit. If you are hunting relatively open country, put it 3" high at 100 yards, you'll be pretty close to POA at 250, a top of back hold on larger deer (I live in the north where deer run close to 18" top to bottom of chest) will do the job to a full 300 yards. If you are using a spitzer and the 2 shot method you'll be good for a bit more (which is what I normally do). Keeping less tension on the mag spring seems to make a more consistent long range shooter, and 2 rounds always seems like plenty to have on top.
With the same cartridge in my 14" Contender I've had one shot kills on deer and antelope beyond the 300 yard line, so if you know your rifle you should be good to at least 300.