Replacing values in one column with values from a second column based on a third column Using R-Studio

So I have a list of 2500 places that I've tried geocoding. Roughly 97 won't geocode. However a while ago someone went through by hand and geocoded these. I've left joind the newest list of geocoded places with the old hand-geocoded table. I'd like to replace the missing/bad geocodes with the hand-geocoded records. The data looks something like this

NewLat OldLat Flag
29.019 39.213 1
41.23  41.23  0
NA     38.13  1
0.00   41.29  0

I'd like it so that the following happens:

If the NewLat is NA or 0.00, replace the value with the OldLat If the record has a Flag of 1, replace with the OldLat.

so far I have

df$NewLat[is.na(df$NewLat)]<-df$OldLat

but the second part is stumping me. I've tried

if("1"%in%df$Flag){df$NewLat=df$OldLat}

and elseif (df$Flag =1) {df$NewLat=df$OldLat}

and mutate(df, df$NewLat = ifelse(df$NewLat<1.0,df$OldLat,df$NewLat))

but nothing seems to be working.

Any suggestions?