FTX Vantage stripped screws

[Triniboy]

Guys

I recently bought a FTX Vantage from ebay but it looks like the previous owner ran it through the rain and the muddy water may have resulted in the screws around the gearbox being locked in. When trying to unscrew the gearbox to change the spur gear the allen key is beginning to round off. How best to tackle screws when the allen end end are beginning to round. I was also just think of buying a new one and using this one as parts, its just the spur gear I need to replace.

Cheers