The simple answer is your GEL battery has more than enough to crank your 29HP engine. You should have around 700 cranking amps at 32F and this will be closer to 900 cranking amps at 70F.. Look for wiring or voltage drop issues. An AGM, unless TPPL, will only get you a couple hundred more cranking amps at 32F....

If you still want to convert to an AGM starter the best product to keep the AGM start battery topped up from a GEL bank is the Sterling battery to battery charger.

http://www.sterling-power-usa.com/ProCharge-B-waterproof-dcinput-battery-charger-12volt-12volt.aspx