Easiest fix is to replace the bad Mach amp with another Mach amp.
99 Mach systems have three amps. One behind/below the radio, and one on each side of the speaker pod in the rear (Vert amps are under the top storage). The front amp runs all four 2" paper "mid-high" speakers. Each rear amp runs 2 5x7" speakers. If all of your amps were good, you could replace the deck and the whole system would play. However, if you just replace the 5x7" speakers and leave the rest factory, they will only play bass since the amps driving them only put out bass signal. If you remove the factory 2" speakers and replace them with a real tweeter, you will lose lots of midrange signal since the factory 2" speaker plays much lower than a tweeter can.
So...You can replace the bad amp and be done. Or, you can at the very least buy a deck and 2 pair of speakers and bypass ALL the Mach stuff.
Alpine deck $100+
Alpine 5x7 components front $99
Alpine 5x7 coax rear $79 (or you could run components in the rear since you already have the Mach pod which is set up for components).
An amp will make the system sound much better but it is not essential.