I have worked on a ton of those rifles, the trigger box is meh, you can get away for a short time with replacing the trigger sears, but they will eventually break forcing a full trigger unit replace.
For the hop-up/inner barrel, you can replace the inner barrel with an off the shelf barrel for a Maruzen L96, the hop-up unit leaves much to be desired in terms of percise adjustment, but put in a high quality rubber and it should work.
In the cylinder, to get any long term reliability without getting slam fires at the bare minimum you will need to replace the piston, stick the high end brands such as PDI and Laylax for best performance.
Spring guides, cylinders, cylinder heads are all preformance boosters, but should not break on the gun under normal use and you shouldn't need to worry about them for now.
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