Help! Honda GXH50 won't turn over

I just bought a Honda GXH50 to use with a Grubee kit. Before I took the stock exhaust, tank & governor off I started it and it went first go and ran beautifully for a minute. Then I stripped it for the Grubee kit but since then the crank won't turn over and the rope pull stops dead when it engages the crank after 4" of pull. The motor is stuck totally unable to turn over.
I thought it was hydro-lock caused by fuel or oil getting into the cylinder but it's not that cos I've taken the plug off it and still no use.
I took off the rope pull which was all fine but I couldn't turn the fins to make the motor move at all.
I haven't a clue now so I think I'll take a look inside where the governor spindle goes into the engine to see if something fell off or is stopping the engine from turning over.
Any advice would be gratefuly received.
Fosscati is flummoxed!

Keep us updated on that one....

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