Oh yeah -- if the D-104 has its original crystal element, is it still good? And since most BITX models can be wired for an electret mic, have you put a DC blocking cap in the mic audio like to the D-104? Eihter of those will cause a D-104 not to work as intended.
This is one of those things where it's probably easier to experiment a little than to work out the mathematics.
The ubitx wants to see a low-impedance drive from the mic, comparable to a 600 ohm impedance or thereabouts. The D-104 crystal element presents a 50K ohm impedance, and the amplified D-104 presents an unmatched impedance that's not really compatible with any 600 ohm that a low-impedance audio input wants to see, except that the amp typically overdrives to the point that something works.A 50K - 600 ohm audio transformer between the D-104 and the ubitx would probably straighten nearly everything out. A 1000 - 8 ohm audio transformer stripped out of an old pocket transistor radio or CB would probably be close enough.