I’ve replaced sever traction tires on LGB engines. Looking a a pic of a LGB Forney I just don’t see a problem, you would first remove the side rod which would be removing the screw at each end, now you should be able to get the new tire on, only do one side at a time, this will keep the wheels properly quartered. You may also have to remove the slide piston rod as it could also be in the way. Since the old tires have come off the wheels, or if loose just cut them off. I do find that putting the new tire back on can be a problem, but I use a stiff 1/8" piano wire rod that has a small 90 degree at the end which is about 3/16" long and filed to a flat taper, this will allow the bent end of the wire to slip up under the new tire and help it around the wheel. You will also have to hold the starting place of the new tire as it will try to come off. This with the engine frame in front of the wheel is not easily done, but doing it slowly you can get it on there with the wire tool. Also removing the engine block from the engine is also another possibility, but without the engine in front of me I can’t see what to tell you. Good luck, it can be done, using the wire it works like a shoe horn and you are lipping it around the wheel.