Complecated compared to what a garden tractor?
Tunning diesels is slightly different than tuning gas engines, and these are different than say a Cummins because they are IDI engines.
But you need to give them lots of cool air and lots of fuel. The stock IP's can only deliver so much fuel, thats the first limit. So you spend $3k and bolt on a Myna IP, now you can deliver more fuel than air, so you need a big Holset turbo and intercooler to keep the EGT's down, of course bolting on a better exhuast in the process.
Than you run into design limitations. Thats why if I was going to go nuts and build one of these I would start with a 606. The 606 has a 4 valve per cylinder head so its going to produce more power.