Hi Md, I would use super glue. The PE simulating the wiring harness will not be load bearing or handled once in position, so you should not need the extra strength of epoxy.
An alternate method might be to use fine copper wire to simulate the wiring harness. It would be a challenge to drill out the harness ring, which looks like it is simulated on Part 31. Drilling spark plug hole in the cylinder heads would not be a tough. I build mostly 1/32, with an occasional foray into 1/48, and that is how I always make the wiring harness. Again, 1/72 might be too small to pull that off, but if it is not, the time spent making the wiring harness using the technique above is time well spent in my opinion.