looks to me like it never had an oil change, and cheap oil if it ever did, I will try to find some pictures of the BMW engine, suspect they were on an old work computer...
If all you want to do is a compression test, just do it.
However cleaning wise, I'd be inclined to get it running, fill it with a 75/25 mix of oil and diesel, run it around 1500rpm to get it warmed up, drain it, and repeat a couple times. If you have time let it sit for a day or so in between draining and refilling, to let the diesel soak in to the sludge further into the engine a bit. Then remove rocker and sump, and give everything as good a clean as you can.
Then once you've fitted a new turbo, make sure you check there's a reasonable return flow from the turbo.
Then change the oil and filter maybe every 1k a couple times, or until you're confident the oil isn't just filling up with soot from the sludge.
If you want any semblance of reliability afterwards that has to be a full stripdown. All that gunge will be in the oilways, round the bearings and is likely what caused the turbo to fail. It all wants steam cleaning, blow the oilways out with compressed air and rebuild it with new bearings, rings etc.
If you don’t want to go that far then the method for cleaning it off is unimportant, just scrape, brush, bit of kero, bit of this and that etc. Its unlikely to run for long anyway unless done properly.
Looks like it wants chucking that to me! I'd be taking the sump off first and inspecting the oil pick up, crank and big ends if they're accessible before I do anything! I'd be concerned about oil starvation in places because that engine hasn't had it easy. Unless of course your planning on flipping it then scrub the worst out, get it running and shift it on...
sump plug out, oven cleaner and fill it up , and then again, then wash it out with diesel, then sump off and send the oven cleaner upwards, put it together with cheep oil and see if you get oil pressure
I ve used oven cleaner getting a inlet fanimould clean before !! never an engine ...perhaps coca cola would be as good ? cant see the sence of oven cleaner if your going to take sump of to clean anyway but maybe its a thing ,interesting