As far as i know, Matt (MHR) is the only one doing a 310 kit.  LRD used to do them back in the day and im sure he would do one again for the right price.

LT250r's are under rated.  They can be built up to make good power comparable to a trx.  Front clips can be changed to handle as good as a new bike.  They are everywhere and you can find them at a low price.  87-92, are the years you want, avoid the 85-86 as the aftermarket parts just arent there. 

My .02