just a quick 1 running a 120 70 17 on my fzr6 3eh got a 110 70 17 h rated on a spare wheel for my 3en1 are these tyres interchangeable as quick cheep fix as it says a stock 1 sould be the 110 anyway as this will be my doner bike for the 600 conversion in the darker nights

should be fine if it's 110 stock in terms of size, will be differnt to ride and may not match the rear so well if its a diff. make.
my mot guy likes both tyres to have the same speed rating but i'm not sure it's a legal requirement.

The 3en1 and 3he wheels are interchangable as long as you use the right spacers. It's only really the disc dia that are different on the whole front end

If your front wheel size is 3 x 17 then you are fine with the 110 but if you have a 3.5 x 17 then I would stick to a 120!

The wheel size should be cast into the wheel either on one of the spokes or on the edge of the rim near the tyre.