If I were you, I'll have my nozzle tips (injectors) checked first if their opening pressure & spray pattern are still within specs, adjust valve clearance to specs install OEM air filter and then install OEM or good quality fuel filter. About the 2T, it has been discussed many times in the other thread that the best ratio is 1:200 to get the best performance out of your engine. So in your case 50ml 2T will be mixed to 10L diesel. You can delay changing our T belt at a later time.

BTW, you might also have a stuck-up brake caliper / cylinder piston that is causing your advie to be sluggish. You may want to check them too