Streido i was just looking at the kymco parts catalogue you uploaded and its part No. 12391-KEE1-900
so i assume we should have one but i have not had to remove my rocker cover yet so can not say for sure mine has got one but i would be very suprised if it did not, As you say the previous owner may not have replaced it as i think its probably rubber rather than cork so more vunerable to heat, may have fell apart when he removed the cover and so he just chucked it not realising the important part it plays. I have just looked on ebay i searched (139qmb gasket) it returned a seller selling one for £6.50 but for a extra pound or two there are sellers selling full gasket sets which includes head and exhaust gaskets etc which may work out better value as im sure the other gaskets would come in handy in the future....