Oh there is so many options even for Africa.
Why you want to crystal lock it to 50MHz I have no idea.
There are Soooooooooooooooooooooo many options.
ok, starting with a ch0 or ch3/4 modulator out of an old video recorder.
An OM350 or rather two of them stripped from old masthead amplifiers can be used as QRP amplifiers.
2.4GHz TV transmitters are disgustingly easy to build. A simple common base oscillator is all you need with a little bit of pre-emphasis.
The problem there is bringing S band FM video to baseband. The cheat is to make it L band (about 900-1800MHz) and use an old analog satellite receiver.
There are LOTS of those in Africa.
Ah Dave, don't you know about my cache of misc ATV circuits.
Lots of mischief there...
This is an example of a common base oscillator that can be used as the basis for a microwave FM ATV transmitter.
http://home-and-garden.webshots.com/photo/1535422598048941318NLMheI
But there's no reason why you could not just build a simple VHF or UHF TV transmitter.
This one transmits about 300mW and will send a signal a couple of kilometres with an external antenna
http://home-and-garden.webshots.com/photo/2229740740048941318spQqHK
But if for some freaky reason you can about the quality of your signal and demand it be crystal locked, then the above circuit can be driven
by a butler oscillator rather than a free running oscillator.
http://home-and-garden.webshots.com/photo/2020691270048941318MjnSgk
The best part is that all of these components can be stripped from any old TV, even and especially a turret tuner
Africa is a place of raw materials if you know how to use them