91 Merc 25 - Should I try to rebuild it?

[jonduck]

Got this thing about a year ago. Ran fine for a while but then started having intermittent slowing of RPMs going full out. Got worse last weekend so my buddy and I took the cover off and when checking the plugs found that the top spark plug had no gap left as the electrode was all of the way bent down; (Yes, they're the correct NGK plugs so it's not an issue of using the wrong plug.) We swapped in a new set of plugs and again had running issues at upper RPMs. Pulled the new plugs and, although not bent all of the way down, the top plug electrode was bent so I assume there's a bad bearing or something allowing the top piston to travel too far.

It still starts well and will run at idle/low speed and I really haven't it pushed it much past 1/3 throttle since the second plug bent.

So it's essentially a parts engine unless it gets rebuilt which I don't want to pay for on a 26 year old motor.

I have lots of tools and have always done my own work on my cars and also all of the work on the 20 or so motorcycles that I have owned, including building my own custom bike from scratch so I don't think the wrenching part of it is an obstacle.

So my questions are:

Parts availability and cost.

Any special tools I might need that are expensive.

I figure that worst case I can't rebuild it and learn something in the process or best case I can rebuild it and get a running motor back.

 

[racerone]

Easy to do.-----Take it apart to evaluate and see what can be done.----Some have a chrome bore and not much can be done there.-------E-bay for parts on a budget.

 

[Canufixit2]

Any chance the top of the piston is well carboned up and hitting the tang on the poon?