What's your 20? You might be amazed at how lardy some engines can be - My old Suz DT25 (supposedly lightweight 2- stroke) clocked in at a mere 7Kg lighter than my current 60!
Assuming it's a new honda 50 you are thinking of, they are remarkably light for a 4- stroke. As are some of the new Yammies. But as Pol says, if you are out Orkney way, there may be an argument for living with a few extra Kg so you have a "20Hp aux" rather than a 4....
There is a fair amount to consider in this twin / single arena - but whatever happens, just remember that what may be true at 300+Hp is a whole
different kettle of fish at the 50-ish HP end of the scale
Post back if your searches raise more questions.