I’m planning a trip in US and Canada. I have a Sennheiser G2 wireless system today, but what frequencies am I allowed to use in US/Can? I’m thinking of buying a second-hand system adopted for US, and only to bring my camera and mic with me.

As long as the freq is UNDER 700mhz you'll be OK. Now of course that's not to say that there won't be people using a frequency you wish to use but that's going to happen anywhere with any freq.