Please help my 7MGTE died on me....

[Naomi87]

Hey pplz... I'm new to the site... my name is LEE....and just a 2 years ago I had purchased a 1987 Supra (7MGTE) from my uncle... and I've gotten nothing but problems w/ this car but to reallie sum it all up I had already changed most parts in order to get the vehicle running properly... like.. clutch pump .. new radiator...new spark plugs.. etc...after that the car was running decent.. so I decided 2 start working on modifications like strut bar, new crystal clear head lamps w/ H.I.D.'s...rims...then exhaust(3" piping w/ Apexi N1 Muffler) .. then.. "DOWNPIPE (3" also)"....now here's my problem... as soon as i put on the new downpipe I was getting a lot of back fire when I drive her normally shifting, and it was mostly a louder backfire @ high rpm or when I was boosting her...so I took her back to the shop next day and the owner said it is normal b/c the piping was new....??? so I believe him and a few weaks later she started 2 reallie give up & the engine sputtered A LOT like she was having a real hard time 2 even idle; like she reallie wanted 2 cut -off & A LOT of WHITE SMOKE started 2 come out from my exhaust... and ever since then I didn't know what caused the 2 die on me.. I've asked people around here in queens.. some say the turbo's exhaust seal went bad & all I have 2 do is replace the turbo... other tell me that the head gasket blew... but I know that it didn't b/c I didn't see any oil or radiator fluid all over the engine nor was it leaking under her...??/...So please I could reallie use some expertise on this one b/c I have tried everything...I would reallie appreciate the help... I love this car 2 death & would reallie like 2 bring her back 2 life.....THANKS....

P.S.
Please can you also let me know if my engine modifications were right or wrong...???
Or if anything else can cause this problem..??
Also can you notify me if anyother problems may occur.....
& what I can do 2 bring her back 2 life.....