Hi!
First of all : Take the engine apart and change those ballbearings if you don't have done it already!
An engine that has been sitting for 10 years is more than likely to have old castor oil goo in the ballbearings and that is stopping the crank from moving freely.
I don't say that the engine won't run if don't do the above but it sure will run much better and will hold up much better if you do.
Webra .50??? I don't think Webra manufactured any .50 engines 10 years ago...are you sure it isn't a .40 or a .60 engine? Well it dosen't matter really, as it should still run.
Put on a 11x6 or 12x4-12x5 APC prop and do use a Enya 3 or Rossi 3 glowplug, the Rossi 3 glow plug is what webra uses as their standard glowplug for the .40 engine.
Use 5% nitro, Not more!
See to that you use adequat glow power! A small 1,2 V Nicad is not recommended! A 12v, 12A lead acid MC battery is what I use myself.

Regards!
Jan K
Sweden