help...i think i have a big problem, tell me what you guys think Options

[bslik]

ok check me out, so about 2 weeks ago i had brought my car to a repair shop to fix the bad lower engine knock. the shop owner told me 1700 to replace my 5sfe with a new 5sfe. so i go to pick it up earlier today and he added on all types of additional charges. came out to $2400.00

so as im leaving the shop, i notice the car losing power and eventually it started with the same knocking noise again and turned off at low RPMs... so now im thinking this guy didnt do anything but maybe put an oil additive and not swap the motor. my question is, how can i tell if this is my original motor or not? whats the location of the engine serial number? or just what do u think i should do?

thanks for any help