Apparently, 60dbA is about the equivalent noise level to a conversation in a restaurant.... so not bad, considering the size of the engine. If you've got a few quid left over, you could always add a layer of AcoustiBlok (tm) sheet - this is (apparently) seriously sound-deadening stuff - their 6mm sheet is claimed to reduce sound levels by 32dbA, which would get you nicely below "Library" sound levels (40dbA). Since their website doesn't quote prices, you know it's going to be dear...
I was planning to use this to keep a generator quiet on my transporter; but after I stopped racing some mumbleteen years ago, the project lost any sense of urgency it might have had in the first place...