That would depend on how you want to reconfigure it. Are you looking to upgrade the internal parts for better performance? Or, are you looking to change the outside parts?
An Airsoft gun
You can but it depends on the airsoft gun you have. On some it will work and others it will jam or even ruin the gun.
A pump airsoft gun is a gun that has to be pumped every time before shooting. Ex. Airsoft shotgun
There are plenty of p90 airsoft guns out there. I recommend searching "airsoft" in google maps. Once you find a store, check their website for the gun of your choice. Hope I helped.
No
No
It is an airsoft gun based of the PPSH-41 gun
You can't without some experience in metalwork.
You can pimp your airsoft gun yourself or you can ask some help from an airsoftsmith nearby. The most important thing in pimping your airsoft gun is YOUR CONCEPT, internal and external. Without a good concept, pimping your airsoft gun is such a waste. What style do you want? A government issue M4A1? Taliban's AK-47? Gunslinger style GBB? PMC's AK SOPMOD? Or just an usual AEG with craziest ROF and fps ever? It's up to you. You can try pimpmygun.doctornoob.com. This website can help you to design the look you want for your AEG/GBBR.
You never need a license to have an Airsoft gun.
No, it is an airsoft gun.
No