I can’t promise that it can be done on a 254 but I have done this with my 654. First you need to drain he oil and remove the sump. Then, remove the clutch and the engine case behind it. Then you can pull the main shaft that takes the drive from the crankshaft to the clutch. The starter clutch sits on this shaft. As you pull the shaft out, the starter clutch can be removed through the sump.
It was not too hard getting it out, but very fiddly getting it back in. Probably even more fiddly on a 254.
As I say, I can’t be sure ths will work on your bike. It depends on whether the 250 is just a scaled down version of the bigger engines.