D-jet is indeed an open loop system, but there is almost nothing to tune except fuel pressure and idle speed. Set these once, and that's it. The problem is that it is fairly complex, and 30+ year old parts are less than reliable, some being hard to find.

K-jet is very simple -- I say fix it, or have it fixed (and there are mechanics who are familiar with it), and it should go for many years with no attention. Any carb conversion using new parts will run $500 or more; it's cheaper to pay someone to work on what you have.