it will be eather spark or fuel. I take it you put new plugs in the rebuilt head, if you used champinis chang'um. 1000 miles is all they are good for. mite look for any thing hitting in the dist. rotor to cap, pick-up to relucktor. but it does sound a little more like fuel to me. check flow , run the carb. line into a bottle and see how fast it pumps gas. if you've never done this before BE CARFUL!!!! and it has to pump a lot more than you mite think. for the frist timers on this if you think OMG! this thing is pumping way to much, it's about right.
again, BE CARFUL!! fire can runin your day fast! naxt would be carb, don't care how low the miles were. does it run better when cold? I mean if you start it and as soon as you get oil presser put it in gear and drive off hard? better or same? my brain haerts
just some things to think about.
your foolish helper.