Don't lock yourself into Motorola specific logic - those are all obsolete and you may get chinese junk.
These boards were made with various manufacturer parts including Motorola, GE/RCA, Signetics and others. Buyers tended to stick to one manufacturer but I have seen pretty much the whole list of various manufacturers on these Williams boards.

For new parts -
For the MC14068B -- the Texas Instruments (GE/RCA) part number would be CD4068BE.
For the MC14050B -- the Texas Instruments (GE/RCA) part number would be CD4050BE.
Don't spend more than 50c each - which is more than the going price for new parts.

The board is wired for an 8-bit DAC even though they only used 6 of the bits on some boards.
For this - the new part is Texas Instruments DAC0808LCN (8 bit) and is often dual marked with "MC1408P8".
These are typically in the $2.50 range.

Yes ^^ what schwaggs said.

Divide and conquer. Inject your own signal into the amp. If that works then the problem is upstream from the amp. If it still cuts out, problem is either the amp or downstream from the amp.