The MK4 turbo is useless, its VNT, has no waste gate, you would need to fit all sorts of complex pulse vac systems to make it work with custom management... you just wouldn't bother
So, its either fitting the AFN turbo... is it actually any different? Its only another 20bhp which the 90bhp engine will do on stock turbo anyway? If you only want 110 bhp... just remap stock engine.
Or... Fitting something else, So, you either need to find any bigger turbo that physically fits in the hole, and buy/make adapter flanges, or find something else that would fit, as its a KKK Turbo, its not like its Garrett, which use similar flanges, so often fitting different turbos are a straight swap, but i don't think you will really find anything that will directly fit that would also make any difference.
Tbh, you only really need a remap, as the KKK turbo on the car will push a steady 1bar in with little lag and holds it pretty well, bigger turbo will make higher boost, and it will hold it longer, but .... with a rev limit of 4000rpm... you dont need to hold it, nor would a relatively high compression diesel engine want more than 1bar unless you get those pistons out.
My advise... Keep the turbo, fit the injectors, and custom map to those injectors, remap will increase boost to its max level, and turbo will handle it pretty well.
