Vassago
Ah geeze mate dont buy from ebay. Only buy gen parts from ebay US. You can get a yuki/SH3135 for USD 195 bucks excluding shipping:
http://www.yukiwatch.com/index.html
do note that there is some cavitation damage on the rotor. you can polish the rotor with a fine grade sand (high grid number) paper or using a drill with wool head and fine dialux vert (green). If you want to polish shine then get a dialux vert (orange). Basically the more fine the polish the more shinny it will be.
Also note that the balance wheel is plastic for some yuki movements. Yuki movement also suffer the problem of peeling purple gears (3135-540) at the auto rotor winder. My advice is to change those with gen from ebay as well while you are at it.
In general the movement is ok but can be serviced easily.
You can make a SH 3135 to a monster with gen parts and some luck.
I made 5 hybrid gen movements and never had any issues there after.
As to your questions, i had tried to migrate the SA3135 parts to SH3135. It failed in many parts due to tolerance and design. Not a single parts could migrate over except the balance spring. SH3135 is a better setup for a gen movement. The SH3135 yuki comes with a stem by default, however due to the extra stem length you need to cut off with
End Cutting Pliers
and grind rough edge of the stem to suite your crown. you can remove the SA3135 stem from the crown using one pliers. The crown is hold down with the stem with loctite. So slowly remove the stem by rotating clock and anti clock wise. Do not use flame to heat up the steam and loctite, the flame will damage the second seal lock in the crown.
Please measure accurately before you cut the yuki stem. Also use loctite 243 at the thread near the crown to secure it. The loctite is a glue that slow drys.