Up until 2018 OTK seat supports were welded onto the frame.
From 2019-2021 they replaced the support with a sort of rectangular steel block, with two holes drilled and welded in a vertical positicion.
From 2022 onwards this block is welded in an horizontal position.
Judging by the serial number, the frame at least should be from around late 2020-2021, but since OTK’s from 2019-2021 were the same (OTK would have updated its chassis lineup in 2021 but COVID changed everything) I would be more concerned about cracks, the tubes not beign excesivelly flattened, and the straightness of the frame.
About the Rotax, the engine should be from 2020, the engines I have from late 2018-early 2019 are 862XXX. Be aware that Rotax compared to Vortex and X30 has had many updates over the years since the introduction of the EVO series back in 2014-2015, in your serial pic the engine has the old style bendix cover, not the current one that covers the bendix and clutch drum. It should do no harm to check the accesories just in case, at least the wire harness, ECU serial must be code 666 815, the spark plug cap should be preferably either the red NGK or the red Rotax one, the black NGK one is not legal anymore.