One of these will do the trick:
EBay item no: 121300016481
It's a copy of the OE tool (as, indeed, are the Sealey and others). They ship pretty quickly; I ordered mine on 27 July, and it was here a week later. They had marked it as a commercial sample for Customs purposes, and there was no duty or VAT to pay.
I'm not convinced, though, that the range of measurements (between, presumably, maximum and minimum) given in the instructions is right for the 113 engine. Theirs only spans about 18mm, while the MB measurements for the OE tool span 31mm. I'm off to MSL on Monday for a gearbox service and dyno run, though, and while I'm there, will get them to top it up to the full mark using the OE tool, and that will give me a datum to use.