There are a number of sheep drenches available which are from the Ivermectin group (clear wormers). I have used Oramec at 2x the sheep dose and always weighed my goats just before drenching. You can also get individual drenching syringes for either 20 or 50ml doses - good bits of kit to have if you have to drench kids and adults, incl billy goats who are much heavier.
I would not necessarily think that you can translate an injectable straight into a drench, and goats should always be wormed by mouth rather than injection as per recent advice from John Matthews. He argues it has been shown to be more effective compared to injection. I have always preferred drench to injection for the goats anyway.