The generator/alternator certainly looks like a GE unit (and maybe more like an alternator than a generator, but I’m not sure), with the characteristic triplex auxiliary drive gear set. (That arrangement was also used by AEI and Heavy Electrical Bhopal.)
The first batch (of 20) of those Tanzanian 88 class MLW MX620 locomotives (ordered when it was still EAR) had CGE GT586 main generators. The second batch (of 15) might have had alternators, but right now I cannot find definitive information. As a cross-check, the second batch of the Malawi Shire class MX615, built just after the second Tanzania batch, had alternators whereas the first batch, built just after the EAR group, had generators. So alternators may have been standard for the MX-series by the later 1970s.
So I doubt that fitting a GE FDL engine in place of an Alco 251 and retaining the original generator/alternator would have been too difficult. For example, an early version of the GT586, the GT586A2, was stated by GE to fit either the Alco or Cooper-Bessemer engine. That the rebuild programme involved 15 locomotives suggests that it might have been the second class 88 batch that was rebuilt.
I imagine that the power output as rebuilt was about the same as the original, namely 2150/2000 hp. Given that the 1-C-C-1 wheel arrangement was retained, I’d infer that moderate axle loading is still a requirement. The originals had an axle loading of roundly 29 000 lb, so much more than 2000 hp might make them too light-footed.
Replacing the original 12-cylinder engine with an 8-cylinder engine was logical given the progress in per-cylinder outputs since the originals were built. The GE FDL-8 had reached 2000 hp by the early 1990s, as evidenced by the U20C 8-cylinder export model (not to be confused with the much better-known 12-cylinder U20C, which was one of the stalwarts of the 1960s and 1970s when it came to dieselization in the CMT gauge world.)
One could ask why not use an Alco 8-251, given that the basic engineering for its installation had been done by MLW for the MX615, which used the same frame as the MX620. But the 8-251 was quite rare – in locomotive service anyway - although Africa (Nigeria and Malawi) may have been the biggest market. On the other hand, the FDL-8 was very widely used, particularly in Africa. I suspect that the FDL was anyway the better of the two engines, and that the difference may have been more marked in the 8-cylinder case. Apparently 45-degree vee-8 engines are not easy to get right from a balance and vibration viewpoint.
This Tanzanian rebuilt fleet is a remnant of what was once a worldwide fleet of 732 diesel-electric locomotives with the 1-C-C-1 wheel arrangement, built over the period 1950 through 1978. Collectively they make an interesting study of contrasting designs and reasons therefor, and cover the gamut of running gear designs from worse-than-awful to excellent.