have you thought about building a plank on frame tug? of The Cervia. That's the same one that is in masterclass - plans by Vic Smead - and it is a straightforward build, but, the only thing you will need to alter is the MFA belt drive gear box as you cannot obtain them brand new nowadays from MFA directly - however - Ebay is a good source. Rather than using the gearbox you could use direct drive 555 motor from Component shop. running on 6 volts and that will you ample push for towing etc. Also, you will be able to take it down to next year's Mayhem Bash and enter it into the competition.
Just a thought for you my friend.