My advice, based upon my own experience is to fit the inspection cap. I had two UFMs I was working on at the same time several years ago. One was off the ex-Cecil Mills bike which I had used as a normal road bike for many years and the other was the oil tank I had used on my production class sprinter. That oil tank was a second hand one of unknown provenance. I worked on the ex-CM one first and spent several weeks with petrol, paraffin and even a pressure washer working down the filler neck. I was still not happy so I then designed a 'man hole cover' got a chum to make several and cut a hole. It was just as well that I did as the solidified black residue left in the bottom of the tank could only be removed by actively scraping it. This experience encouraged me to then make a hole in the top of the tank of my sprinter. Absolutely clean! Such is life. The one thing that I do remember is that the run of the pipe inside the tank was very different between the two tanks so when cutting a hole in the top be careful not to go too deep.