Here's how to DIY remove and replace the valve springs, keepers, retainers, seats and seals on a Honda K20 or K24 engine. Links to the specialty tools below.

Lisle Valve Keeper Removal and Install Tool: https://amzn.to/2A7k7rN
Valve Stem Seal Removal Pliers: https://amzn.to/2AdDi32
OTC LeakDown Tester: https://amzn.to/3nYRvpj