Is it my turbo 1991 fourtrak tdx


I purchased my first 4x4 a 1991 fourtrak Tdx about 1 month ago. After a few days i noticed it seemed to be under powered and took it to the local garage where they said it could be the fuel filter. They changed the filter which did turn out to be very dirty, i also put some injector cleaner in the tank. Unfortunatley my fourtrak is still under powered and i've recently noticed oil leaking from the underside of the turbo area and dripping down onto the alternator. With regards to the turbo it seems as if it works intermittently, sometimes you hear the turbo whistling at about 2000rpm but most of the time it dosen't whistle and there's quite a lot of black smoke and not very much engine power. I have read on this forum about the green turbo light Unknw Do i have one of these on my fourtrak because if i do then it dosen't work as i've never seen it working Unknw I'd be very grateful if anybody could shine some light on how i can get my fourtrak back running as it should be.