Can someone tell me what's wrong?

A guy brought me a bike to try to fix today
Use to run but then got harder and harder to start and seemed sluggish I figured maybe the plug

he bought two of the short plugs one an R10 and the other an R12(don't know the brand and it's too hard to get it back out for now)
Anyway put one plug ion and still got nothing
Put the other plug with a longer thread and the rear wheel locks up completely when I dump the clutch

That's all I had time for today

I had a feeling that it was a too long spark plug causing the lock up
He only did it twice so hopefully no damage was done

Compression was good, at least as good as any of mine

I don't really want to get too deep into this thing I never set myself up as a mechanic and am not charging anything for this
it's just a friendly gesture from the gas bike community to a kid that is starting to feel that all of us are rip off artists

His problems mimic my son's and his was caused by a fouled plug

Forget to mention that fuel is coming out of the exhaust, more so with the new (Shorter) plug, than with the old

This suggest to me that it is lacking fire??

Next step is to loosen the friggin engine again, because the builder put the head in backwards for the bike and the plug wire can not even be removed in that position and put one of my stock plugs
I may call and ask permission to turn the thing around right

If that doesn't do it I'll check the CDI wires, I have experienced that before also

I know it is a lack of fire somewhere

OK so it's not the plug length. Held it up to a stock plug and there is no difference
Plus I took the plug out and the engine is still seized

Could this be a clutch issue unrelated to the hard starting issue

Any suggestions, anybody?
This thing is locked up solid with the piston in the up most position I have taken bothy the plug and muffler off along with the head and nothing is moving
With the head off the piston jiggles from side to side in the cylinder, so that is not bound up

Could this be a locked up clutch?

The kid needs this as his only transportation to his job that he has had to take off for two days now

I have an engine for an OCC build here but I would have to charge him about $100 to switch it out

Yeah pulled the clutch plate and it turned freely
It has to be something inside

Does $100 sound like a good price to switch out an engine?
If he wants it on the road quick that's our only option

yeah $100 for everything, plus I'll keep the bad engine for parts and the learning experience

I saw an engine only for $84 plus shipping