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Most QRP CW transmitters get by with just the output low pass filter
because the oscillator is working directly at the desired transmit frequency.
There is no mixer to create spurious products.
The only problem is harmonics, and the lowest harmonic is double the desired frequency
and thus easy for the output low pass filter to remove.

A rig with a mixer such as the NE602 at IC5 will have undesired products.
Other than both input frequencies and the sum and difference frequencies,
there will be additional spurs due to harmonics of the input frequencies.
For example, when tuning this rig to 3.6 mhz, the VFO will be at 8-3.6=4.4mhz.
The 5'th harmonic of that VFO at 5*4.4=22mhz will mix with the third harmonic
of the 8mhz local oscillator at 3*8=24mhz to give a spur at 24-22=3mhz.?
That spur is probably 30 or 40 dB down, but still loud enough to make the rig illegal.??
Being lower than the operating frequency, the output low pass filter will have no effect.

The problem with the Rose-80 is that it uses sharply tuned resonant circuits?
at T3 and T5.? On most rigs, T3 and T5 would be broadband,
and there might be a 3.5 to 3.7 mhz bandpass filter (see EMRFD for recipes)
immediately after the mixer at IC5 to remove those nasty spurs.

T3 and T5 are built to be 10.7 mhz IF transformers,
they probably have an internal cap to resonate at 10.7mhz.
The Rose-80 adds an external cap to each to bring the resonant frequency
down into the 80m band.? So removing that external cap is not sufficient
to disable the resonance, T3 and T5 would have to be replaced entirely.

If going to an si5351 vfo, I would remove the mixer at IC5 entirely,
and just drive the final at the desired frequency directly from the si5351 during transmit.

But as I said, the rig is probably just fine as built.
Adjust T3 and T5 to best hit the band slice of interest and leave it be.
Unless you are compelled to tinker and learn, in which case have at it.

Jerry, KE7ER