intrepid 1999 3.5 engine/1997 3.5 swap

I recently bought a 1997 dodge intrepid at a car auction. The seller advised that he had installed a new 1999 3.5 24-valve engine to replace the original 1997 3.5. The check engine light is on but the car seems to run great, I've had no problems with the car at all. I'd appreciate if anyone could advise what obvious problems occur with this swap, especially what I might try to get rid of the check engine light. I suspect I will end up having a mechanic go through and look at the codes to see what is wrong but would like to see if anyone out is familiar with this particular engine swap

the only difference between the '99 3.5 and the '97 is the design of the intake manifold and that the '99 uses an aluminum block, while the '97 uses cast iron; that's about it. There's an easier way to see why your check engine light is on, turn your ignition key on/off 3xs ending in the "ON" position. The check engine light will flash. If it flashes twice then pauses and flashes another 4xs, that's a code 24. The last code it will spit out is 55 meaning end of message code.