All 9.8's (whatever they were called) used Merc's 10.9 cubic inch block, and throughout it's life the block was never modernized. It was a "headless" split block with the reeds located in a (donut) around the lower portion of the crank - a real pain if you had a reed problem - had to crack the block to get at them.
The 9.8 started it's model run in 1962 as the model 110 with Merc's Phelon ignition system.
In 1970 it got the first "thunderbolt" ignition system - the "thunderbolt 2 - Phase Maker".
In 1974, while still called the Model 110 it got Merc's Thunderbolt 4 ignition which stayed in production as Merc's near perfect ignition until 2005 when most 2 strokes were discontinued.
In 1979 it simply became the 9.8.
In 1985 both the block (which was shared with the 7.5 horse) and the 9.8 model were discontinued.
1986 saw the launch of the 6/8/9.9 horse models with a new 12.8 cubic inch block (designed and built by Yamaha exclusively for Merc) which combined with the T4 ignition became the best and most reliable 2 stroke portables ever to hit the market.
The 9.8, and it's little sister, the 7.5 built post-1974 were a relatively trouble free, generally reliable motor.
However, you have to consider that the "newest" one out there is now 25 years old and some parts have been long discontinued.
In the used market there is little price difference between an early 80's 9.8 and a later 80's 9.9 - with the 9.9, just from a parts support perspective, being the much more desireable motor.
But if you have a 9.8 (or get one really cheap), it could be a viable motor for many more years...