There are a number of bolt on belt drives for sale Bruce, and reasonably priced.
People's attempts sometimes fail for a reasonably simple reason, the failure to understand that engine's 'pulse', they slow down, and then re accelerate every cylinder's cycle, so the crankshaft doesn't turn at one constant speed, besides flexing, while the propeller wants to and does continue at one speed. It is this fight that causes the bulk of the issues. Allowing them to run at their separate speeds, such as flexible drive or one way clutch between them, goes a long way to resolving many of the seen issues.
I once resolved an issue with a large solidly mounted, single cylinder engine driving a hydraulic pump that was constantly tearing out the fiber drive isolators, by mounting it using rubber engine mounts. They were positioned specifically so that the engine pulses would rotate the engine back and forward around the crank's centerline (like many cars), hopefully dissipating the driveshock and TVs. It worked so apparently it did.
They originally didn't use rubber mounts for fear the movement would tear the drives, a valid thought, but it was what was doing the damage.
More cylinders and more flywheel weight go somewhat to lessening TV problems as well.