Before doing java part, the script(class one) block in rpk needs to be added.
Like this
</FILE>
<FILE 00000151.res >
typeof 8
superid 0x0002F23C
typeid 0x000000B9
alias kit
isparentcompatible 1.00
</FILE>
<FILE 00000151.rsd >
script parts\engines\Einvagen_Duhen_Ishima_Focer\scripts\kit_Mitsubishi_engine.class
</FILE>
typeid can be anything from 0x00000000 to 0x0000FFFF and it must unique, by unique i mean there should only 1 entry for that id in the rpk. after you add this kind of block in the engine's rpk, you can proceed in making ur kit java.
As for java of the kit, ALL THE PARTS MUST BE IN CORRECT BUILDING ORDER, if you f*ck up the order the engine won't come complete from the crate.
Hope it helps.