DON'T jetwash it. Better to use some degreaser and wipe everything down by hand. Takes time but worth it. I know some folks have used a small hand steam cleaner. Finish with aerospace 303 on the plastic bits. If the plenum, coil cover and rocker covers are corroded, there's not much that will get them back to life apart from removal and either powder coating or DIY rub down and spray.
Pics before and after please
Ooops - just realised you have 159, not 156. Hopefully somebody with some experience of the 159 will be along and be a bit more helpful than me! Perhaps jetwashing is an option on the 159?