you can get a complete Crosman 357 on amazon.com for like less then $50 shipped
and thats everything except the "specific" parts that make it a aga.62 (I.E the barrel, elevator section, feed tube and indexing armature,barrel latch) but you get a brand new(better I say) valve, combat grip, co2 puncture area etc.
The only downsize is you have to mill a bit off the bottom of the "swivel part"(the bottom screw area) as the aga and the 3357 have larger parts on their front barrel block.
oh and btw if you take a tippmann detent and cut it a bit it works great as a 3357 or aga latch rubber.(cheap-o fix)
now as for the downsizing conversion? oh man for the money it sounds like you might have to drop into that working properly you might as well go with a 3357 in general.
I dont have the machines to make something like that but I can imagine getting it working properly on someone else's time would add up quickly.
Echo is spending a lot of time and his own $ to make his 3357 shoot .62 so I can imagine that a downsizing conversion with an elevator system would be rather difficult too.
not 100% sure just what I would surmise