No need to convert between metric and imperial. It is a ratio of 32 parts gas to 1 part oil, and it doesn't matter how much actual volume the proverbial part contains. You can just carry an empty beer can with you. Dump 8 beer cans worth of gas into your tank, followed by 1/4 of same beer can filled with oil. So the only trick is accurately measuring 1/4 of a can, which is not hard even in the jungle - all you need is a straw which you'll fold into quarters and then use it as a measuring stick.
Wanna get fancy? Well, if you really want to top off and then add the necessary amount of oil, here is how. The idea is to make yourself a calibrated dipstick as follows:
1) empty the tank
2) add exactly 1 liter of gas
3) insert dipstick, remove dipstick, and mark current level
repeat steps 2 and 3 until the tank is full. Now you've got a dipstick which tells you exactly how much fuel you have in your tank. BTW, you'll have to insert the dipstick at an angle to reach the lowest point in the tank. It is not a problem, as long as you insert it at the same angle every time (i.e. just jam it into the back of the tank). Oh, and you have to keep the bike leaned the same way every time too. When you get to your jungle fuel stop, take a measurement before and after you top off, and the difference tells you how much gas you added. Now divide that volume by 32, and that's how much oil you must add. I recommend you carry a medium sized syringe calibrated in cubic centimeters for measuring oil (remember that 1 liter = 1000 CCs). BTW, dividing by 32 in the jungle without a calculator is not too hard: just divide by 2, then by 2 again, then by 2 again, then by 2 again, and finally by 2 once more (that's 5 divisions by 2). Or to make your life simpler, just remember that you need about 31cc of oil per 1 liter of gas (1000 / 32 = 31.25)