Hi guys,

Just picked up an old ag175 2 stroke bike for cheap, apparently they got it started not long ago but was blowing a lot of smoke.

I mucked around with it a bit, planning to strip it all down, but tried kicking it a few times and it's like its jammed at the top, until you put it in like 5th gear and bump it slightly, then it'll free up, but is stiff as hell and impossible to get a full push on it.

What would cause it to be locked until a small roll forward with it in gear?

Hi & welcome. Not enough info to give a worthwhile guess just yet. Think you'd better look further first then let's know a bit more. Could be a belly full of fuel or fuel & oil from gravity fed oil tank with & minor pump fault causing 2 stroke oil to fill the crankcase [but unlikely].
Take plug out & then the magneto flywheel cover off & revolve the flywheel by hand or socket & crank handle speed wrench thingy, but have ignition key off just incase it sparks & sets fire to everything with stray fuel out the plug hole.
Bottom line is just see if it turns. If you have moved the kickstart down the do that again & bring it right back up, otherwise if the kick gear hasn't fully disengaged then if you are winding the motor backwards on the flywheel its likely it'll hook the gear & stop you turning any more [backwards only that is] so could give you a false idea of whats happening [or not happening].
Everything being normal you should be able to crank it in both directions [by speed wrench that is].
Doesn't sound hopeful though!!.
Hope nothings come adrift in the magneto but if the kick lever is free to return & only tight to push down & turn the motor, i think its cylinder off time for a start, or clutch cover & cylinder in that order & if nothing obvious in the clutch cover like the crank nut has come off the gear & is pushing into the case. Enough confusion for now.

Thanks for the reply. Hopefully this weekend i'll get some to have a look at it,

I pulled the oil pump? cover off yesterday and it was basically filled with creamy sludge

I have noticed one of the screws holding the right side crank case has the head stripped out of it, so might have to buy a tool to extract that too.