As you have probably guessed it's hard to crimp the replacement rivets without the proper tool. I do it with a large pair of gas pliers and a ball bearing. The pliers provide enough grunt to splay the steel rivet and you use the ball bearing on the hollow end - it evenly spreads the end so you get a neat crimp. I think the ball bearing is around 5mm diameter but I have of course lost it.
HTH
Brian