8HP Evinrude 2-stroke YACHTWIN (25" shaft) weighed approximately 64#
the long-shaft model (20") weighed 58# and the short-shaft (15") weighed 56#.
I suspect the outboard bracket will hold the weight of the new NISSAN, but I agree that if this is the original mount..... there is no time like the present to replace it, for piece of mind and because the new one could have stronger springs making it easier to lift the outboard while sailing. A new mount is not cheap, but is still a lot cheaper than repairing/replacing an outboard that fel lin the water when the old mount broke.
I don't buy the theory that one MUST replace the mounting bracket ONLY because of a switch to a 4-stroke engine, the original brackets on most of the O'DAYs (early 22 and 20 not included) were rated for a 15hp engine (figure 70-80#), so I can't see that a 4-6hp 4-stroke will harm the mounting bracket. But when talking about replacing an 8HP, 64# 2-stroke with a 9.9HP, 80-100# larger 4-stroke and factoring in the age of the bracket.... well, replacement is justified.