x = data.frame(Índice = c(1,2,3,4,5,6,7,8,9,10),
            Data = as.Date(c("2019-01-22", "2019-01-21", "2019-01-20", "2019-01-18", "2019-01-17", "2019-01-16", "2019-01-15", "2019-01-14", "2019-01-11", "2019-01-10")),
            Valor = c(20,25,28,42,59,18,72,63,18,42),
            Busca = c(59,18,0,0,42,28,0,18,0,50))

x$Encontrado <- 0

x$Encontrado[x$Busca>0] <- x$Valor[x$Busca>0]
x
#   Índice       Data Valor Busca Encontrado
# 1       1 2019-01-22    20    59         20
# 2       2 2019-01-21    25    18         25
# 3       3 2019-01-20    28     0          0
# 4       4 2019-01-18    42     0          0
# 5       5 2019-01-17    59    42         59
# 6       6 2019-01-16    18    28         18
# 7       7 2019-01-15    72     0          0
# 8       8 2019-01-14    63    18         63
# 9       9 2019-01-11    18     0          0
# 10     10 2019-01-10    42    50         42