I'm afraid the reverse sounds are actual samples and have been manipulated in a wave editor to be a mirror image of the original waveform before being made into the ROM. Thus the envelope settings for them are no help in substituting other sounds since you need a sample with a waveform decay at the beginning and a reverse transient at the end, the exact opposite of all the normal drum samples in the machine.

For simulation you would need a constant bass type sound and adjust a slow attack and peak with a high sustain 2 envelope. Swamp it with enough reverb and a suitable dsp and you might get something to pass muster in a mix with other things playing.

I would be inclined to use the sound edit menu rather than the drum edit because there you have a valuable delay available enabling a second sample to be superimposed at the end of the sound, time aligned with the reverse envelope of the first sample. You could use an actual drum sample with its envelope attack and decay adjusted to disguise the origin of the sound on this second sample, preventing it sounding too 'normal'.