Unless you have a lathe i dont recommend making it wider. Buy a new pulley. Its like 2-5 euros.
1:3 in gearing is nice if you have a single motor build.
I have tried to bore out a motor pulley before. It is very difficult without a lathe and did not work well. Unless you are desperate, I would recommend something else.
I would recommend buying new pulley’s because I had the same issue. I drilled the pulley out with a drill press and that worked out for a few rides, eventually the pulley just shattered.
If you have access to a lathe or know some one who does, no problem.
If not, its hard, but can still be done if you can use a drill press. First, you need a good drill press, none of that wobbly misused abused and unmaintained equipment. Also you need a weird size of drill, DO NOT use 10mm, as the hole it leaves will be closer to 10,1 mm and your pulley will be pretty much useless. Drill the pulley with 9,5, or better 9,7 or 9,8mm drill, then you need to use a reamer to finish up the hole and bring it to 10mm.
Now that I wrote that, if you have access to a good drill press and reamers, you probably have a lathe nearby too… You can probably just buy a new pulley and save yourself the time