Okay, without  getting too detailed:

You ideally need an Oscilloscope good to 100Mhz with 10: 1 probes and an FFT display in math. Also a good 50W, 50Ω load for testing the power output. A DMM is expected as is a variable amplitude & variable freq. CW signal source.

To build the better Amp (the hacked one) use the LTspice schematic in the files link to match component values to the supplied PCB silk screen parts. Note the BOM text file as well.

Note the logs below detailing an alternative build mod#2 that improves the purity somewhat esp. for 40 meters. THE Mod#2 LTspice file carries all the details there.

Now I HAD to swap a 2n2222 in for the input transistor as i accidentally killed the OEM unit while experimenting. 9V across the base-emitter can do that 8(. Note the 'dead bug' installation of the 2n2222 in the pictures

You can try it out without swapping the input Tranny Q3, it should be fine as is. Your input gain beta might be a bit higher though.

Also note the wire jumper installation replacing the T1 transformer and the 470Ω through hole resistor used to 'jumper' the Q3 base to SMT R4. That through hole resistor can be substituted by a 510Ω OEM SMT part if u solder wire leads to it for mounting.

This allows you to try both emitter and source follower  Q3 configurations by just moving the wire jumper from the collector to the emitter.  I found either worked similarly. Note the R10 feedback is removed altogether ( becomes the 510 Ω thru hole DC bias resistor) as is the C16 HF gain compensation and the T1 transformer.

A couple DC bias & feedback values are changed for the 2SC1971 Q2 in order to improve its spectral purity. This draws around 0.6A of current at 14.5V and heats that transistor , so be sure the 4-40 hardware, insulating washer and insulating pad are installed firmly to the heatsink. A 120mm Fan (80CFM) mounted on the H.sink fins  is required for max power use over more than 5 mins.

The L1 RFC, T2 and T3 transformers are wound as follows:

1) T2 = 7:3 turns ratio (pri: sec) of the thinner magnet wire supplied. 4:1 inductance ratio

2) T3 = center tapped 3 to 7 turns (pri:sec) of the heavy magnet wire. 1:4 inductance ratio

3) L1 RFC is 9 to 10 turns of the heavy magnet wire.

Having a current and voltage controlled linear PSU on hand is helpful when first powering up to prevent over current damage due to improper assembly errors.

The J9 solder pad jumper on the bottom needs to be shorted in order to use a single  12-15Vsupply.

I'd suggest setting the supply current limit to 1A if that is available to you. Otherwise a 10 Ohm 3 to 5W power resistor in series with the supply might be in order. You can test the voltage drop across it should not exceed 6V when 1st powering up. If that is good you can remove it and apply direct power. Otherwise check your build for shorts or reverse polarity on the transistors or 1Kuf electrolytic.

Calibration Discussion:

Be sure the Transistors are well mounted and insulated from the heat sink before doing this step.

The key item here is RV1 & RV2 need to bias the MOSFET gates to  Vth which is about 3.35V for the IRF530 supplied.  Anymore causes current drain & heating. I have observed that once Vth is reached the current drain goes up.

RV1 should be turned fully COUNTER clockwise before starting, RV2 should be fully  clockwise before starting so that the Gate to ground resistance for Each MOSFET is a few ohms, measure with your ohmmeter b4 applying power.

Apply power.

You can CAREFULLY bias each FET slowly using RV1 & then RV2 watching the current drain on a 0-10A setting of your DMM. Once it goes up (from the  quiescent half amp or so)  by 20 or 30mA per MOSFET you can stop raising the gate voltage. If you go too fast you can spike the current thru the FET and blow it with a non current limited supply. A reasonable current limit is around 3A here for safety. Eventually expect to draw a bit more than 5A at full bore power.

Applying an Input Signal

With the 2n2222 input stage I found a 5Vpp max signal works well. This is more useful than the original 1.1Vpp max signal when driving with a QRP signal source.

You can start with a 0.5Vpp and work up, use around 14Mhz as the working frequency for optimum spectral purity as shown in my scope images.