ORIGINAL: DarZeelon
Until a major spare part is needed...
...Then, you are better off just buying a whole new engine.
In other words; it is a disposable engines, use and discard...
Just like an OS. Their parts are similarly priced as well -- & you are quite correct -- it isn't worth making a major repair to either one. Fortunately neither OS nor Thunder Tiger need much in the way of repairs.
The only cheaply repairable engines in North America are the PRC OS clones.