[Kelvin]

The main questions to be asked (some have been already asked)are:
(1) How large is that field?
(2) Are you planning on using a rotary, flail, or finish mower?
(3) How wide of a cutting path do you want?
(4) How rough is the terrain?".

In my opinion...
Most small tractor's PTOs will handle larger mowers than they are rated for. This is mainly because of stability issues on slopes. If your terrain is relatively level then Bird's setup will most likely do just fine. If you have steeper terrain, then you may want a tractor that is 4wd and/or larger (heavier in the front end). It won't necessarily have to have any more horsepower.

One recommendation would be to buy the size & type mower that you want, then use it on your L35. Based upon that experience you should be able to hopefully judge how much less of a tractor you could stand.
Kelvin