As David Haywood stated "The ST-V cartridge for Decathlete contains a chip marked 315-5838, this chip provides encryption and compression related functions for the game"
The issue with this game is that it makes extensive use of decompression to such an extent that almost the complete graphics would be to need decompressed and then adapt the game code for it.
@Darksoft discussed it with @MetalliC and this doesn't look like will be doable anytime soon.
as was said above - this game have most of graphics/textures compressed, also they are stored as separate "files" (not grouped into bigger data chunks).
so, there needed to extract/decompress hundred(s) of files and then rebuild game data, or implement in SH-2 code on-the-fly decompressor.
any of approaches is not easy and will require fair amount of work...
I understand, I thought it was easier with just an encryption to break.
Thank you for your explanations
Is Decathlete that different from the Saturn version?