Despite what has been posted by others, and no dispute as to the validity of said posts, but I would try the bike with the GS ignition first before going to the trouble & expense of trying to find an MX ignition. Our bike was bone stock (GS ignition) except for a custom exhaust & muffler (modified CR250RZ pipe/KX500 muffler). It made good strong, useable power in the hands of an intermediate rider and actually won quite a few races against comparable opposition. The bike will have plenty of power, so I would recommend you determine you need the extra oomph of the MX ignition before you buy one. The money may be better spent elsewhere. My Husky CR430 was too much for me with the CR internal rotor, flighty and tiring. An external rotor WR ignition tamed the bike and it became much more rideable and enjoyable for ME. Try yours first. Just my opinion.
Cheers,
K