either your "stock" motor ain't making enough power or there's something up with your second gear. it'll shift to second around 10 - 15 normally. but when you say you can whine it out in first then it's probably something up with the clutch. inspect.
Take apart your second gear clutch apart. notice the wear where the spring clip that holds 2nd onto the shaft is beveling and the wear on the clutch bell, something is binding it up, rubbing, loose etc. Make sure you have all the right washers/thicknesses, location. I wanna say by memory there is a thick washer before clutchs go on.
if thats missing or the spring clip is missing i suspect something is missing,
while you got it open, clean on the inside of pads, kinda tarrry somtimes, balance, check pins not damaged, right orientation, etc.