Hi - to ONLY re-torque the headbolts, you can go buy a torque wrench and correct socket from Sears, Home Depot, Lowes, Harbor Freight Tools, etc for about $100.
Torque wrenches are simple to use - just read instructions.
You should also buy a Yanmar
manual so you can get the proper torque values and the proper bolt tightening sequence. Ydou might be able to find this on-line at some website.
HINT: Torque the head bolts in 2 steps. If it says that the final torque should be 50 foot pounds, set the torque wrench for 40 foot pounds and tighten all the bolts, then re-set the wrench to 50 foot pounds for the final torquing.
If the factory manual givens metric or Newton torque values, you can find conversion values on numerous websites to change them into foot-pounds.
If you have to replace the head gasket, you should probably buy a "head de-carbonizing kit" or "valve job kit" which usually has all the gaskets that you will need to remove and replace the head.
You may need a couple more metric wrenches and sockets to do the job, but you should have those tools onboard anyways. You will also need the torque wrench.
If you have to pull the head, it would be a good idea to at least have the valves lapped or lightly ground. A good machine shop should only charge about $100 to do this.
I don't know how much Yanmar parts
are, but the metric wrenches and sockets and a engine
manual should cost around $100-150.
So for sockets, wrenches, torque wrench and manual maybe $200-250 for all the tools you will need to remove and replace the head.
engines are NOT that hard to work on. Just read the factory manual and go step-by-step. The whole experience is a really important skill set to have for cruising.