Siff wire the size of the holes is what is required.
You will get a new key with the tumbler, I think -- check, and get one if it doesn't automatically come with.
Get rid of all your aftermarket brass keys, too -- they wear and will give exactly the same problem. I've seen steel keys worn enough to give trouble, but you've got about 15 more years of daily use to get to that point!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!