Polini Thor 250
It is a good idea to check Polini's website to get the latest (or additional) information.
Carburetor (not a Walbro) 28mm throat diameter, main jet 118 (according to Polini but it could be something else). These carburetors are more like the Bing.
Note Polini updates their parts frequently so these lists may not be 100% accurate.
Spark plug – Champion RN2C gap 0.023"-0.028" (0.58mm - 0.71mm) Polini does not specify the gap value in their engine manual. The value here is from Champion's technical site. I suggest that pilots use the lesser gap value because of this engine's high compression.
User manual (single and dual spark plug)