Congratulations Pete, good first journey was it solo or with support car?
As for radiators the one on Bumble is a 3 core with a standard fan. If I was going to buy a replacement I would go for either the 2 or 4 core, heard all the grasstrackers use & love the 2 core but they only do short stints, at full chat, but not for a long time like a road car so not sure which to recommend. Is there a thermostat in & if so does it open up well. Mixture & timing will have an effect, does it have antifreeze in, bit bizarre question asking in summer, but anti-freeze lifts the boiling point & allows the water to flow better. Water wetter is another product consider, also fitting a metal hot country fan.
But all that aside, my advice would be, get timing & mixture checked (rolling road best), change the radiator (to your preference) fit a heater matrix to keep your feet warm (or put it at the back) it also increase the total capacity of the system. The finally I would install a expansion bottle, seal the top of the radiator, feed the small pipe from the radiator into the expansion bottle & tee the bottle into the heater matrix loop.
Alan...