My guess is that the service will be v. expensive and probably not cost effective on a 2nd hand engine. I’d be tempted to run it up in a bucket of water to see if it runs ok and to check for some exhaust/cooling water spraying out the back somewhere. My Suzuki has 2 little holes on the shaft facing backwards and the spray comes out of them. I run mine in an old bin. It’s possible to remove the prop first (take out the split pin and look for the shear pin behind the prop – you don’t want to loose that) – taking the prop off stops all the water shooting out of the bucket. But I usually don’t bother. Don’t over rev it – without the prop there is not much load and it is easy to get the engine screaming with only a little throttle. If no cooling spray then kill the engine before it heats up – if you let it get too hot it will seize up but you should be able to run it for a minute or two.
If no spray then either
– broken/damaged impeller – fairly easy to replace
– Blocked cooling channels in the head/block – Head needs to come off to clear them. Mine has two sets of channels (one set you get to by removing the head (where the spark plug goes in) and one set (the exhaust or leg gasket – I don’t know the correct name) you get to by removing the shaft – Probably not for the faint hearted but it’s certainly very easy to do on my Suzuki. You’ll need new gaskets and make sure you chase and clear the channels all the way from one end to the other – it’s easy to miss bits – it’s incredible how they can get fully blocked and become almost invisible.