Hi Icecream, welcome to the board. I have heard of people succesfully transplanting the x1/9 1500 engine into a 128. However like you say they have problems with the oil pick up, possibly due to the engine in the x1/9 being slightly tilted causing oil starvation. Possibly fitting a Strada sump will correct this? I havent got much experience with engine transplants myself, however, I have done a little bit of research for you and you may find the following website usefull. Hope this is of assistance.
https://www.fiatforum.com/other-fiats/86133-fiat-128-engine-swap.html
https://www.fiatforum.com/other-fiats/86133-fiat-128-engine-swap.html