The Yamaha RX100 accessories are out of the list from most of the after market shops as the originals have stopped being produced a long way back. The custom accessories are readily available whereas it is rare if you get a new one in the market. The fuel tank can only be taken from some other RX100, which should be good in condition but will surely require a repaint. The carburetor would not be available as the stock one, but you will easily get a closely related carburetor from a aftermarket shop. The scarcity of the spare parts have lead to the heavy charges by mechanics and you should try to find the part yourself to save up to half the money being charged by mechanics. The cost is most of the times dependent on the seller as the part is mostly used one. Try to keep the budget as low as possible as spending on a vehicle is not always profitable at the end.