If you don't want to buy any other mottled chickens, cross a Malay to your bantam hen. All F1 chicks must carry the mottling gene.
After that, you've got some choices:
--You can breed F1 chicks to each other, to get about 25% mottled chicks. Pick the most Malay-like of the mottled ones, and cross back to Malay again.
--You can cross an F1 chick back to the mottled hen, to get 50% mottled chicks. Pick the most Malay-like of the mottled ones, and cross back to Malay again.
For either of these, you can just keep repeating the same steps, but after the first time you use a part-Malay bird with mottling rather than the original hen, so you are increasing the amount of Malay in the mottled birds.
--Or you can cross an F1 chick back to Malay, to get 50% that CARRY mottling (but don't show it). Raise a number of these, then test-mate with a known mottled chicken (like their mother) to see which ones carry mottling. Cross one of them back to Malay, again getting 50% that carry mottling and need to be test-mated to see which ones. This requires a lot of test-mating, but may overall be a faster way to get as many Malay genes as possible in a bird that also carries mottling. At some point, cross two mottled-carriers to get ones that actually show mottling.
I've read that mottling is sometimes visible in the juvenile plumage of chickens that carry it, even though it won't show in the adult plumage. If yours do that, you may be able to pick out the carriers of mottling without having to test mate them, which could save a lot of time. From what I remember reading, it was partly determined by what base color the chicken had: I think it showed more on Extended Black than on some of the other e-locus alleles. If you have several colors of Malays, you could try several options and see which (if any) work to let you spot the mottled-carriers as chicks.