Without a doubt...
WEST-System epoxy.You don't want an 'adhesive' here anyway (though you probably know that). An adhesive glues two things together. What you want here is a bonding agent for wood-saturation and lamination, which is exactly what most of those tillers are put together with.Scrape out as much rot as you can. Dewax/strip with acetone (soak several times each overnight). Make it as clean and dry and close to bare wood as you can. Test for 'clampage' --if you squeeze it with a clamp, will it close tightly? If not, you MUST fill it with a piece of wood somehow or the epoxy will just dribble out and do nothing. If it does clamp tightly, wet out both surfaces really well and clamp it-- give it h*ll with the clamps too.WEST epoxy works by doing one of 2 things:1. it enters the very pores of the wood, sealing off all interior areas (volume?) so that when cured it's like a web or a lot of fingers locking mechanically into the wood;2. it bonds with itself later, tying into epoxy resin already locked into the wood on both sides to act as a kind of glue.But WEST EPOXY IS NOT GLUE!!!! Without the first part done, the locking into the wood, putting epoxy on two pieces and clamping them together will fail to hold it. The epoxy only glues to itself, not to clean wood. Whatever you put on first will creep into the wood leaving nothing left to hold the two boards together.Your tiller is probably already made with epoxy. So the first part-- the saturation-- is already done. If that is the case you have only to chemically soften it up, using acetone as I said, and apply the second part-- the 'glue' function to adhere the two parts together.If by chance the tiller is not made with epoxy (like if it's from before about 1973) then using the same glue they used then is the answer.I'm betting it's epoxy. Try it and see.JC