I would bet that one of the lifters is plugged up. What I would do before I spent a bunch of time and money on it is to flush the oil system out.
Just get some Kerosine/Diesel fuel and mix it 50/50 with new motor oil. Leave the old filter on. Drain the old oil out. Fill it to the full mark with the mixture. Start it and run it just above a low idle until it is good and warm. Usually on an air cooled motor that would be 10-15 minutes under no load.
Then just change the oil and filter like you always would. I have had this fix many, many hydraulic lifter noise issues over the years. I had a 1952 Chevy Pickup that told you to do that in the service manual every other oil change.