There are many methods, but you are super smart to do it yearly...avoid too big of resi-doo buildup.
From my perspective, as a former refinery engineer, you might consider something in a multi-step cleaning.
The Wild Lion is all mild steel, correct? Safer to clean than stainless, but a tad less easy.
My old cleaning method from the 70's was to soak the inside with a mix of diesel and a general engine solvent. No need to pack it full, but you must "agitate" the contents quite a bit. You know, shaking it up and down, back and forth or if you can, vibrate it a lot!
We did this on the race bikes, and to save money, drained the solution through a coarse filter into a container to use again and again. When you think it's goo free, you can just put it back on...or spray a good amount of cheap oil like WD-40 inside to prevent rust. If storing for winter, you MUST protect the steel from oxidation. If using again, the un-burned oil will coat the metal.
The hard part is that you have what I believe is a baffled chamber, so you have funny flow-angles inside, making it harder to clean, therefore do it and do it again.
Recently, we go the extra expense to porcelain coat the inside of our pipes, making the cleaning super easy due to less accumulation. For racing it is the bomb! Slick inside and less deviation of wall form. Inside welding defects will "hang on to" oil molecules, due to high temperatures, they just keep clinging on.
Everyone knows that Cling-ons are not good. TV, movies and your dog knows...
If you have stainless, then you must always refrain from using water or water based solutions. (I tend to use cheap vinegar to remove rust...but never from high temperature metals.) Stainless tends to retain the sodium content in liquids, as it will dry in the defects. When it gets into the 340+ temp zones too quickly, the molecules will create micro explosions from pores and super minute cracks. Each time it takes away base metal. After a while you have holes or general thinning or both. You have junk!
I can show you my original Gyro poop-shoot. It has a hole just next to the stinger, from inside corrosion. It is very common in Honda scooters with baffled MFFLRSSS...ding, ding, ding.
The OEM exhaust picture shows hot spots from heat/corrosion exposure. I use cheap paint for just that purpose as it degrades quickly with temperature. Easy to clean it off later, but effective in understanding temperature relations.
The darker spots are cooked paint, which happens quicker in "hot" zones. Next to the stinger, was black, so I put a screw driver into it when hot...went right through, because base metal molecules were barely holding hands. (Not in picture)
I am sure there are many cleaning methods available today, but the history can be worth a new cash outlay for expensive exhaust and more tuning.
