Depending on your budget, needs, and engine building skills..
There is only one choice for a inexpensive bolt on kit- the BBR 120. Add a real cam (as your funds allow) Web 68/69 or MegaCycle 129x6 and KW HD valve springs, be sure to check all your clearances- Maxim exhaust valve lift, with out sinking the valves or valve guide mods is .285") figger about $200 upgrade in addition to $230ish kit price= $430 Kit will still have a low 10.5:1 CR.
Carbs on the cheep!
24mm 200x ATC
These will bolt on the stock intake and air box boot. Use the stock XR200 or CRF150F throttle and cable. and/or the CR85 throttle and cable.
I've been testing/running a PWK 28 from a KX100 on my- far from stock BBR kit! 35 pj, JJH, 115 mj.
With all the mods above- the XR100 might just crack double digits for HP#'s
For about $600-700 budget op for a supper head 115 kit from Kitaco or Takegawa- will be solid in the teens for HP#'s as hot as you can get with out adding a stroker crank.
FS MINT 04' CR250R Enigne Complete