cr 250

hi i am intrested in buying a cr250 1986 of a mate, i am a car machinic and have done a bit ov work on 2strokes ie. topend rebuilds etc but i not that greatly clued up on them, the bike i am intrested in isnt running, it has had a new piston rebore etc, it was runnin for about half an hour after rebuild but was running poor and keep cuttin out and gettin harder to start until it wudnt resart, we have checked the rings, bore, headgasket, carb(jets float), reeds, and we have a good spark but still cant figer it out, so my mate sent it to the local bike shop and they had a little look at it for an hour. the shop said that ther wasnt no fuel gettin into the bore, he didnt take no more farther action cus he said he could be working on it for a while to find the prob and fix it, a few pep have been saying it could be the crank seals. (this is were my lack of knowedge comes in) cud this cause it not to run? and also the side caseing behind the break lever has a crack and is leaking oil, would this cause it not to run? ie cud ther be a loss of vacum persure witch would cause fuel not to go in? could someone shed some light on this as if it isnt goin to cost me to much to fix myself then i shall buy the bike off him cheap, hope to hear back with some helpfull advice please.
thanks ian

also i just rembered when kicking it over we are gettting oil coming out where the exaust joins to barrel. cud this be a reason for it not to run? but the shop didnt say nofin bout it he just said ther no fuel goin in

aprently it was smoking quite a bit. but i dono if it was excevie cus i wasnt there. but the oil seems qute cleanish, i dont see how it could be unburned mix because every kick it drops oout like 3 big drops for each kick roughly, so would the fact that the sidecasing leaking wouldnt cause it not to run? thanks ian