My MX60 on my barge was always a bit smokey and I never got it as clean running as the smaller MX40 that I had on a previous narrowboat so I can't suggest a guaranteed cure. However some experience of working on my MX60 is as follows (bear with me because it is a few years ago since I last worked on one):-
1. IIRC the heater plug holder is a very tight fit into the body of the heater and, IIRC, uses a heat resistant sealant when reassembling. Have a look at an MX60 manual for guidance. The heater plug was so tight on mine that I had to make a special puller to extract it cleanly which I left on the barge for the new owner when I sold it in 2017. Trying to lever it out just bends the retaining flanges, unlike the MX40 which is so much easier.
2. The wicks are a very tight fit into the body of the heater. You have to carefully align the hole in Wick A with the heater plug holder hole while pressing the wicks into place. You should also get the heater plug holder gasket (Part 21) if you do this job
Overall I found the MX40 easier to work on than the MX60. There is an adjustment on the MX60 IIRC that allows you to alter the air/fuel mixture ratio but I never had the required gas test equipment to be able to investigate that. That's what Shims 27, 28 and 29 are for IIRC. You might be able to get a gasket set for it to do the job but ask Mikuni about that.
If you find all this a bit daunting there used to be a contributor to the forum IIRC who was an accredited service agent for Mikuni but I don't think I've seen him post for ages to the best of my knowledge.
The Mikuni Heating UK technical guy that I always found very, very helpful was Steve Elliott on 023 8052 8777 (Again, it is some years ago, maybe 2015 - 16, since I spoke to him so can't be sure if he is still there)
Hope this helps.
Roger