If you have a header already on the engine almost any pipe used for a tag engine will work. As far as the carb not sure since I’m not as familiar with the rotary style FA engines. I think this forum you’ll find many people that could help you out with this question: http://www.karting1.co.uk/forum/
For a pipe the TAG pipes will work great or the original ICA/FA/FSA, as far as carbs ICA used 24mm purpose built carbs tillotson style.
FYI-If you are looking for a ICAor FA type engine I have a Parilla TT37 rv and a PCR rv both 100cc and I have some 24mm ICA type carbs. I can offer the engines as is or completely rebuilt same for carbs.
You are looking for carbs that are the “HL” pattern. Typically they come in 24mm and 30mm venturis. 24mm being more common and easier to tune. A TAG carb will suffice, but a nice Ibea, baroni, Hubchen or tryton work nice. The carb that came with the Sonik TX125 (Vamec) is not a bad one for the money at all and since that motor is basically obsolete you can probably pick up for buttons. TAG pipe will be fine too. Vortex ROK is probably best to a match.
Enjoy the simplicity of the setup. They are a hoot to drive. Simple, light and very nimble. Run between 8 and 10oz of castor oil/gal. No syth with an aircooled!! Leaded gas is kind to bearings and gives an extra layer of protection so use that if you can. Keep peak revs under 18,500 and you’ll get a long time out of it, assuming you are starting with good parts.
If you’re on facebook lookup the groups “Proper Karting” and “F100”, lots of 100cc cronies on there.
Motors wise, try and get anything that doesn’t have a bridge in the exhaust port. In other words, you want something with an oval port exhaust. Most of those motors will be 95 onwards, the exception being the rotax DSC which was 1993.