I am new to dirt bikes so if this is a stupid question take it easy on me! I am looking to buy a used Yamaha 2019 YZF250. The owner says the bike has 90hrs on the engine and has recently did a top and bottom end rebuild. Is it common for the 4 stroke engines to need a top/bottom rebuild so early? I thought the 4 strokes were supposed to be a lot more reliable and durable. What is typical maintenance on the 4 strokes?