2000 Honda Accord EX with a 2.3 in it loses power going up hills

I have a 2000 Honda Accord EX with a VTEC engine. I blew the head gasket at 75 miles an hour. Rather than spend the money and have the head refinished. I had a 2001 Honda Accord with a 2.3 but it was a DX, non VTEC engine

Took the whole top end off the DX motor and put it on the EX motor, that is, the head, the intake manifold, the throttle body, distributor. I replaced the wires, the distributor cap and rotor, plugs. Put a newer fuel pump. And I replace the piston rings.

I don't get any codes, the engine runs great idles properly and everything. I get 24 miles per gallon in the city and 30 31 miles per gallon on the highway. I'm very pleased with the performance of the non VTEC head.

But as I said, I lose power going up hills. I've driven it through the mountains from Wisconsin to Virginia Beach taking interstate 81 which climbs are long but they're not steep. Anytime I hit a steep hill I lose power and the transmission tries to compensate for that loss by engaging a lower gear I believe it revs the rpms pretty high sometimes.

Anybody got any ideas on how to fix my dilemma. Oh by the way, I have 506,774 miles on it. I really don't want to stick any more money into it cuz I've replaced pretty most parts. Has a new timing belt water pump the only thing that I haven't changed is the oil pump. I have zero diagnostic ability, all I do is what I learn from you guys, so any help you can give me, I'm listening.