I can only suggest a computer approach.
Because Yamaha files are just re-named SMFs, you can open them in any Midi sequencer. Several good ones offer a 'half-time, double time' option. If you want to have the style play at the correct tempo (although, if it works, why mess around?), you'll need to edit them this way....
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An arranger is just a tool. What matters is what you build with it..!