Hi ,
the first thing you have to do is find the cause of the white stuff : damaged head gasket , broken liners ... on the condition that the white stuff is inside the engine ( on a piston , in the oil sump ...) . Then once you found , to make an engine that will last many miles , you can replace the following parts :
spark plugs , piston rings at the minimum or the pistons/liners depending on the mileages of the engine and the damages you found , all the seals , the head gasket , the timing chain / timing chain tensioner , the main shell bearings , the big end shell bearings , the thrust washers , and check or replace the oil pump .
What you will have to check also is the flateness of the cylinder head and the condition of the crankshaft .
There is a valuable rule : when you work on an engine , work in clean condition .
Good work .
Cheers