Unknown starting problem

Just bought a 2009 crf250 used (50hrs), got told it just had engine rebuilt while it got stroked to a 300. Went to start it for first time, only ever riding Yamaha,ktm this is first 4-stroke. I pulled yellow choke out all the way. Found the top on kick start. 5-min off kicking and no go. Tried with half choke next day, nothing, tried the advice on this forum & nothing. Took off seat, took off air filter & sprayed 1 sec of start ya bastard, put back together. No choke and kicked, it fired for 2- seconds then back to nothing. I’ve had it a week and havnt got it going once. The buyer is reliable but now overseas. Wat can I try before taking it to a shop and spending all that coin?