Baka top overhaul lang yan, change valve seal and change valve guides to be sure walang play ang valves.

Simple lang naman ang 4K engine, if you have a trusted mechanic patanggal mo lang yung cylinder head para makita status ng pinton kung may alog, pag may alog na ang piston dyan na papasok yung gastos sa machine shop. Pag wala pang alog ang piston, sa cylinder head lang ang gastos and usually does not need machine shop, worst case on cylinder head was machine press of new valve guides.

Since you mentioned na nagbabawas ng ng langis at puti ang usok ng tambutso, ibig lang sabihin ay humahalo na sa combustion ang langis.