I know what you mean, but you then have the engine feed to contend with.
There are several differences aside from the lube system of these pumps. I know they upgrade/change R20 with 24y and such, and my info says R11 should/con be replaced with R18, but I have never done either... do they bolt up?-- prob.
do they run right / .. don't know.
However , there is a 113 guy close by here that does know interchangability of these pumps and parts quite well and he does answer questions via email.
His name is Gernold and is a 113 specialist. I am sure he can help you on this one better than I.
Here is his site

http://www.sltechw113.com/sl_question_form.htm