I would just rebuild the old ones. They are alumunum and will not go bad unless they are bent. I am still running the orgional shocks on my 10T.
The bearing upgrade would be for the transmission and both the front and rear wheels. Batteries today are so efficent and powerful that they run at higher speeds for a longer period of time, creating more heat.
The bearing upgrade is a must in my book. With an inexpensive modern battery, and a modern stock 27t motor you should be able to get 15 muinute runtimes (which is almost double from back in 1990). The technology is so superior its not even compariable. With double runtimes and faster speeds everything will heat up more quickly, and thats why I mentioned the bearings.
From the look of the picture you are running either silver or gold springs, which are on the heaver end, because of the weight of the buggy. I mentioned the thicker oil because heaver oil goes along with the heaver springs. To light of an oil and the buggy may feel "bouncy" or "springy" to you. But either way the rebuild is the most important thing.
I have been able to rebuild the rear diff, but I have not been able to find the gears. I am still using the orgional gears in the transmission and they are working like they were new. Just clean them an put them back together.
Though others may disagree I wouldnt run anything faster than a 17 to a 19 turn motor in the buggy, with a modern battery. I keep mine on the quick but not fast side because I want to perserve the old truck the best that I can.