i would like to try a different coil on my sr250 to check if my coil is faulty.i can buy coils that look the same cheaper for the rd 200/250 or the xs750/850 and others.are they all just the same coil?

a coil is a coil if it fits it will work provide it is 12 v or 6 v to match. But that said a bigger coil will normaly have more windings on it and produce a different spark at a different peek inverse voltage. Ie bigger coil bigger spark but you need the coil to charge it with. small charge coil, big spark coil =0 net effect. or - weeker spark

Stay away form cheep coils .. as they say you get what you pay for!! Try and borrow one or Better yet test your coil with a meter.

They are different electrically and just throwing one on may not work. You are obviously aware that mounting points may alter between models.

What may also change is the coil resistance. one may be 200Ohm the next 500ohm. Smaller bikes need smaller sparks. 4 strokes can get away with even less as there is less noncombustable crap in the charge compared to a 2 stroke ( and some 4 strokes i've had ahem). Some 2 stroke coils will be generating 4 times the firing pulses of yours (assuming its not wasted spark). Imagine that at 8000 rpm. 16000 charge cycles a minute.

That generates heat, that needs more mass in the coil to dissapate it so more ohms.

Long story short, google a few easy get options then check out if they are a close electrical match then try them. If there is more than 40-50Ohms between them don't bother.