Mate,
it might pay to calulate the weight of brass engine mounts too
as you might need to add more material too to get the weight up.
if you weigh the AL ones you could do a quick comparison between the SG of AL and the SG of Brass.

SG of brass ~8500 / SG of Al ~2700 = ~ 3.2

so 1 Gr of Al = to ~3.2 Gr of brass for the same size.