Just because you can doesn't mean you should.
You *could* buy all the parts needed except receiver which you purchase in country. BUT how much time and money are you willing to invest?
ie. Why would you buy a JG or something and put that into a G&P metal body? If you were say going to buy a SystemA Magnum/Revolution/Energy gearbox or something and put that in a G&P body it's probably slightly more worth the time and money but you still have the problem of other external parts like stock, rails, grip etc.
SystemA REVOLUTION drop in gearbox ~$450-500 USD
G&P Receiver in Canada ~$200-250 (average price I've observed)
Other parts ~$500
If you tally it up it easily goes way past the $1000 mark for parts alone. Now how much is your time worth to you? Would you rather watch the hockey game, have sex with your wife/girlfriend/mistress/friend with benefit, or do something else more productive than assembling a gun (granted I'm not sure how long it would be to put this together, but for a first timer it usually takes 3x at long to do something like this than someone who's done it a few times, based on my experiences with assembling computers).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE