I have a 250X maier plastics are sweet. the 250x and the 300EX are the same quad the only difference is the bore and stroke and the 300 has electric start. In order for the 300EX plactics to fit you only have to cut out for the kickstarter but if you found a factory set of 300 plastics the kickstarter recess is already there.
I'm putting these on my 250X.. They are for a 300EX sold at www.maier-mfg.com