Rebuild?

Just started my project bike and I think the engine is seized, , I've tried to turn the engine over from the starter motor side using a strongarm bar and socket and it just loosens the bolt, all the spark plugs are out. Even tried putting it in gear with the rear wheel off the ground and turning it still no movement from the engine!
I guess its a full rebuild from here so has anyone got any do's and don'ts for me to start the engine rebuild.

"problems are just solutions waiting to be born"

Unless the engine has been run without oil, I would think it is unlikely that a crankshaft main bearing or connecting rod bearing has seized.  Do you have an endoscope device?  You could look inside the cylinders to check for rust on the cylinder walls, or the possibility of a piston / valve collision due to cam chain tension issues. It would be wise to remove the cam cover in any case to check the condition of the cam chain roller and sprocket, at which time you will discover if there is excessive slack in the chain.

If it was a timing chain piston valve issue, it would still roll over a bit  likely rusted cylinders, sized pistons from a leaky head gasket. Seen that more than once on this site.
spray down the cylinders with penetrating oil and let it sit for a couple of days and then try breaking it free by rocking the crankshaft back and forth. Splitting the engine when it's  sized is very difficult, not impossible, but time better spent freeing it first.
Read the FA Q' s for advice with all the common issues before doing anything.
Good luck.

KB

Just endoscoped the cylinders and they don't look terrible but a bit of surface rust on 4 out of 6 cylinders, and it looks like whoever stored it put a small amount oil down each of the bores, I think best case scenario is just a hone on all 6. Would still love to try moving it but with the rust is that a good idea.? Whether I can free it or not I'll get the engine out and take off the sump to see the extent of the rust. Interestingly there was no oil filter and very little oil in the sump when I drained it.

You still need to break it free if you're going to remove the cylinders and hone them. Once it's free, I wouldn't rotate it more than necessary and then dismantle. There have been others on the site who opted to change the oil and just run it with little consequence reported. That's not how I do things, but to each, their own.. Depends on how fussy you are. I know a few machine shops that just won't fit pistons tighter than .003" clearance because they've had seizures return for rework.  .0015" clearanced pistons need careful religious break-in with respect for the first 1000 miles.

If it is just seized, it will be the rings seized to the cylinder walls most likely. Similar metals (cast iron) in the combination of air and moisture. When they break free, there's no clear clean break so some of the ring material may stay with the liner and some of the liner material may stay with the rings. Normally you would dismantle, hone to clean the surface and install new rings to do it right. As long as the piston to cylinder clearance is within spec you can save it.

My personal preference is a precision plateau hone and if your machine shop doesn't know what that is, then find another one. A standard precision hone is fine, but if your going that mile, then go the extra mile for the plateau hone. It costs a little extra but worth every penny. it's basically a precision hone to within .003"  with a medium stone (280 grit), followed with a extra fine hone (800 grit) to size. What's left is a super smooth finish that's perfectly round for a good match to the new rings with deeper grooves to retain oil for excellent lubrication. This is race engine machining but the advantage for a standard engine is longevity

 In this particular case with trying to resurrect a seized cylinder(s) they would do a quick hone (like 10 strokes) with the medium grit and then follow up with the 800 grit 

As I mentioned in another thread, my motorcycle was stopped near the sea for 6 or 7 years, they told me that someone had removed some spark plugs to place them in an outboard boat engine.
Such a long period without spark plugs and in salty, humid weather caused a cylinder to seize.
The jam was so bad that they broke the engine block when trying to remove the piston.
My bike has a rebuilt aftermarket engine.