This fella look sickly
Posted: October 28th, 2020, 4:32 pm
This fella got some brown and black spots as well as lil bugs. What to do? 
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To be honest I would bin that tree and start again. I know that will sound harsh but it has lots of problems and is probably only worth $20 at the most to replace with a far better tree.
If you are determined to try to save it you need to treat the bugs which are probably aphids at this time of year if they are on the leaves and new shoots. Pyrethrum is low toxic and effective on aphids. You will need to spray 2 or 3 times a week apart to get on top of them properly. Already you are up for $5-$10 for the spray.
Hard to tell what the problem with the bark is about. The brown patches may be sunburn or some fungal infection in which case the bark is probably dead. I am actually surprised the top is still alive. Faintly possible the marks are just on the outside of the bark and the cambium underneath is still OK which would explain that it still has green leaves. I can't really tell what the problem is so difficult to suggest a remedy for that apart from wait and see.
Very sorry I can't be more help. Not every bonsai lives to a ripe old age. Many of us have failures. Keep the pot - wash it thoroughly and leave in the sun for a few weeks to kill any diseases and try again is my best suggestion.