Help needed!!!

I’ve recently bought a focus st225 modded, it’s had a block mod amongst other things done to it, I’ve now discovered whilst changing my driveshaft an oil leak 🙈 at first I thought it was coming from the sump plug so I pulled it out drained the oil put a new copper washer hoping that would do the trick, I drove a few miles and when stopped I checked under the car to see it’s still dripping out from somewhere. Back of the engine is dry seems to be greasy and oily from where the oil filter housing area is.... starting to worry I got myself into a pickle with this car already because it’s also got a boosting problem, once over a certain amount of revs tho it drives like how it’s suppose to other than that seems like it’s holding back.... any helps would be appreciated